Choline uptake across the mucosal border of guinea pig jejunum was measured to determine the characteristics of this step in intestinal absorption. Unidirectional influx of [14C]choline appears to proceed primarily by a saturable, carrier-mediated process at low mucosal choline concentrations; at high concentrations (greater than 4 mM) the influx rate is approximately linearly related to the mucosal choline concentration, suggesting that absorption by passive diffusion predominates. Influx was only minimally reduced by elimination of Na+ from the mucosal test solution or by reduction of the intracellular Na+ concentration. Preincubation of tissue samples with metabolic inhibitors or with ouabain did not markedly reduce influx. These results are consistent with a model of choline transport across the brush border membrane by a carrier-mediated mechanism which is similar to that involved in fructose absorption but different from the Na+-dependent mechanism which participates in active transport of sugars and amino acids. At low lumenal choline concentrations, influx into colonic mucosa is slower than in jejunum and appears to be attributed solely to simple diffusion.