Janvier, Explore's Beninois guide, explained that, once you've consulted a voodoo priest with your ailment, he'll send you off to market with a shopping list. It is, in effect, like a giant branch of Boots. I headed into a booth for a consultation with the fetish master. He showed me various talismans: a stick you whispered into in order to have safe travels; a stick you rubbed on your head for good dreams; and a stick you dipped in whisky which would "make you go like a buffalo all night". Now that's the kind of stick a guy could use.

We left Lomé and headed east, along the coast road, with miles of spectacular white sand and roaring surf to the right, and miles of botanical gardens to the left. We stopped at a voodoo seminary, where we were received by the high priest with that closing-time look in his eyes.

Outside, a man sat manacled to the wall by an ankle chain. "Mad," said the priest. Behind the man, the wall was covered in fresh gore and blood and feathers, like a chicken had recently exploded. Nearby, a woman sat, bowed and silent, under a tree, her feet and hands covered in chicken feathers and blood, a ceremony that will connect her to her dead ancestors.

The priest took us through to a room containing the voodoo shrines: stone deities, mostly, including Legba, the god of fate, whose enormous erection suggested he'd be the perfect poster boy for stick salesmen. Then he led us into a courtyard where a girl, five years old perhaps, perched on all fours, head bowed, staring at the earth. She was being initiated into the sacred voodoo mysteries. Back in the UK, I'd read a report saying that thousands of children in Togo and Benin are handed over to convents by parents unable to pay back their voodoo-treatment debts. We stared at the girl for 10 minutes. She didn't move.

Into Benin, until 1975 called Dahomey, and to Ouidah. Ouidah is the spiritual capital of voodoo and was once a major slave-trading post, where The Viceroy of Ouidah, Bruce Chatwin's novella about a Brazilian slaver's Faustian pact with the mad kings of Dahomey, was set.

We stood in the square, opposite the Catholic church, where the book's opening scene plays out, and outside the voodoo Python Temple. The physical and spiritual proximity of Christianity and Islam to voodoo is found throughout most West African countries, where half the population attends a mosque or church, but nearly all practise voodoo.

Inside the temple, a pit of 41 writhing pythons (it's voodoo's magic number), into which I, no lover of snakes, am invited to climb. "You will be safe," said the python master, who didn't even have a stick, as I clamber down. "They are sacred snakes."

We visited Place Chacha, formerly the site of Ouidah's vast slave market, where today screaming children chase a punctured football, but where once the desperate were led in chains from all over West Africa. Once sold, they were marched off along the Route de l'Esclave, a three-mile dusty track leading down to the beach. We walked along it, too, past the site where the Tree of Forgetting once stood, around which the slaves were forced to march so as to erase their homeland from their memories.

So rapacious were the Dahomey kings in their slave-grabbing that desperate methods were devised to escape them. Several miles out into Lake Nokoué, a village of stilt huts called Ganvie was built (superstition prevented the slavers pursuing them over water), still home to around 30,000 people. We pootled through the labyrinth of canals, over the water hyacinth choking the lake, threading through the market conducted entirely on boats, and past the stilt hospital and schools, the stilt shops and the houses. From the balcony of our guesthouse we watched hundreds of pirogues glide past, propelled by pole, like some tropical Venice, laden with food, firewood, water; kids fishing with nets, or taking the family boat out for a joyride, flirting, larking around on their watery playground.

Over cold Flag lagers, I asked some of our group why they'd chosen the trip. Aged between 40 and 73, nurses, lawyers and meteorologists among them, they were not the sort you'd want to play passport Top Trumps with: they'd been everywhere. A few had an interest in voodoo and slavery but most were simply curious about the world. A nice bunch.

Next day, we stopped in a Yoruba village to see a Gelede mask ceremony. During the dry season, the dances take place weekly to offer up prayers for rain and to ward off malevolent witchcraft. It's about as close as voodoo gets to Hollywood cliché, with wild-eyed drummers in the square pounding out a rhythm and villagers pounding down the palm wine, the local lethal moonshine. Masked dancers emerged from alleyways, some on stilts, dressed as matriarchs with dolls sewn into their costumes, or as Le Créateur, a terrifying 10ft-tall creature with large wooden breasts, or the hyena, with whom parents threaten their kids if they misbehave. The kids, all puffed up and trying out their adult bravery, would flee in terror when the hyena turned on them.

Abomey was formerly the capital of the ancient kingdom of Dahomey, whose kings ruled the region from 1625 to 1900, with an appetite for war, slave-trading and human sacrifice. Two of the original 12 palaces have been restored and in 1985 were granted Unesco World Heritage status. The bold appliqué tapestries in the museum show grisly scenes of heads being crushed in vices and other run-of-the-mill bonkers-king stuff. There was the throne of King Ghézo, who ruled from 1818 to 1858, mounted on the skulls of four enemies, his favourite drinking vessel made from, naturally, a human skull, and the royal spittoon, whose bearer, should he lose his grip, lost his head which, presumably, could then be recycled into a useful household item.

In the courtyard was Temple Zéwa, where Ghézo had a group of his treacherous wives (he had more than 200) executed by burying them to their necks, covering them in red palm oil and leaving them for the ants to eat. King Glélé, his son, left posthumous orders that 41 (it's that number again) of his wives be buried alive with him in a mausoleum built from sand and the blood of 41 slaves.

We headed north now, along unmade roads so hideously disfigured that I thought my teeth would fall out. Outside the jeep's window, a never-ending cavalcade of people on small Chinese-made mopeds carrying livestock (with no refrigeration, everything is transported live): hundreds of chickens and guinea fowl strung around a moped, like a giant boa, with the rider's head just visible above the feathers; goats lying in roped-together pyramids; and a fully grown trussed-up cow who did not look too happy.

We'd stop for a break in the middle of nowhere, and be instantly accosted by hawkers rushing out of the bush selling pineapples, mangoes or avocados, all stacked high on trays on heads. One guy offered me a slice of meat which, although a tad greasy, wasn't unpleasant. He showed me the wicker platter which contained the rest of the animal, strung out, all yellow teeth and tiny claws.

"What's that?" I asked Janvier.

"Agouti," he said. "A rat."

"More?" the hawker asked.

"I'm good, thanks," I said.

We went for a walk through Somba country. Like the people of Ganvie, the Somba adapted their houses to protect themselves from slavers and still build in the same way. Set in the middle of fields of okra or sorghum, the compounds, or tata, consist of a series of turreted towers connected by a thick wall of unfired clay, topped with castellations, with a single entrance chamber where the enemy could be slaughtered with arrows. The Somba still hunt with bows today.

The climate and landscape was changing. From the humid, tropical south, with its banana trees and teak forests, where the red dust kicked up from the trucks made a spectral orange glow and coated the leaves to create a perennial autumn, to flat, endless savannah, where the heat was searing and dry and baobab and acacia ruled. Great herds of lyre-horned zebu scuffed around watering holes and teams of women and children pumped roadside wells furiously.

Across the border with Burkina Faso and, suddenly, gone were the mopeds; donkeys now provided the transport. Togo and Benin are poor, but Burkina takes it to another level. The statistics in my Lonely Planet made grim reading: Burkina is the third poorest country on earth; nearly half of the population will not see their 40th birthday; one in five goes hungry every day.

We drove up to Bani, with its collection of mud hilltop mosques, whose minarets are skewered with wooden slats like some giant Ker-Plunk. It was Tabaskie, the Muslim festival, and a cow lay in the street, pinned down by a dozen men, its life gurgling away into the dirt. And after, out into the Sahel, at the very edge of the Sahara, where we pitched our tents and watched the biggest full moon for 15 years rise into the sky.

The sound of children laughing drifted across the scrub. They led us by the hand, through dry river beds and between the acacia, all lit by the moon like a film set, to their tented village, where the Tuareg were lined up to greet us, eyes of fire framed by turbans of deepest indigo. Behind them their Bella slaves. A fire was lit, the calabash drums came out and the dancing started. After an hour or so, I walked away, far out into the desert, to where I could just see the fire, and sat there, listening to the drums.

On the road back to Burkina's capital Ouagadougou, we stopped off at another village. There was the familiar soundtrack of village life, a thud-thud-thud of what sounded like a diesel generator, but was the women pounding millet into mortars. The village came out to meet us and we were given the tour. The children, in rags, barefoot, chased after us, dancing, smiling, holding our hands tightly. The village chief gave us calabashes of the millet beer he usually sells to keep the village alive. Vultures sat on a wall. The children had the swollen bellies of malnutrition. The chief explained how the village doesn't have enough to eat.

I went off on my own. Wandered among the mud huts and the rubbish. An elderly woman came out of a hut holding a baby, a few days old. Behind her, the mother emerged, naked from the waist down. She looked around 14. She pointed to her baby. I smiled.

The woman invited me into the hut, and there, lying on the floor, in the dirt and the dark, was an ancient man, his rheumy, unseeing eyes looking up. He spoke gently in a language I didn't understand. I put some money in his hand. It felt like such a feeble thing to do. He smiled.

Essentials

The 16-night Cradle of Voodoo tour from Explore (0844 499 0901; www.explore.co.uk) costs from £2,435. Price includes meals, all ground transport, guide, cook and flights. The next trip is in October. Visas are required before travelling. Togo doesn't have a consulate in the UK, so it's best to use a visa courier company such as Visa Swift (www.visaswift.com). Visa costs for all three countries, including courier, are about £240.