Constant, self induced mobility throughout the uterine lumen between days 6 and 17 after ovulation, complete envelopment by a self-secreted glycoprotein capsule between days 7 and 30 and 'injection' of specialised, gonadotrophin-secreting trophoblast cells into the maternal endometrium at days 35-37 are three unusual aspects of equine embryogenesis. The outer trophoblast layer of the allantochorion finally establishes a stable, microvillous contact with the lumenal epithelium of the endometrium around days 40-42 and placentation commences thereafter. The allantochorion elongates steadily until it occupies the whole of the interior of the uterus by day 85. It develops branched, sub-branched and highly convoluted microcotyledons over its entire surface and these interdigitate closely with accommodating endometrial upgrowths (sulci) to provide a total microscopic area of fetomaternal contact for haemotrophic exchange which exceeds 50m(2) at term. Endometrial gland exocrine secretions, imbibed by elongated trophoblast cells that form areolae above the mouths of the endometrial glands, contribute histotrophic nutrients to the fetus throughout gestation. These dual forms of haemotrophic and histotrophic nutrition achieve a high degree of precocity in the foal at birth. This paper describes the gross and microscopic changes at the fetomaternal interface between days 20 and 80 of gestation in the pregnant mare which establishes the extensive, diffuse, non-invasive microcotyledonary placenta that supports fetal growth and development thereafter.