Absorption of the major human bile acids was studied in 12 healthy volunteers by steady state perfusion of the ileum in 112 experiments and of the jejunum in 48 experiments. Use of a randomized order of four perfusions on 1 day of study and use of up to 4 consecutive days of study in a subject allowed important comparisons of data from the same individuals. That there is active ileal absorption of chenodeoxycholic, glycochenodeoxycholic, and taurocholic acids in man was supported by the finding of saturation kinetics and of competition for absorption among conjugated bile acids. Values for apparent kinetic constants (apparent maximal transport velocity [(*)V(max)] and apparent Michaelis constant) in man are similar to those in other species. The ileum absorbed chenodeoxycholic acid more rapidly than its glycine conjugate, due mainly to a ninefold greater permeability for the free acid. Taurocholate had the highest (*)V(max) and was absorbed more rapidly than glycochenodeoxycholate. Passive permeability of the jejunum to bile acids was twice that of the ileum, and the permeabilities to free and glycine-conjugated chenodeoxycholate were in the same ratio as in the ileum (9: 1). Jejunal permeability to chenodeoxycholic acid was three times that to cholic acid. Variation of intraluminal pH by up to 1.4 units did not influence jejunal uptake of free bile acids. These results, which are comparable with those from animal experiments, provide a basis for estimation of intestinal reabsorption of bile acids in intact man.