We have examined the rate-limiting steps involved in bile acid absorption across the unstirred water layer and lipid cell membrane of the jejunal mucosa. Uptake of the polar bile acid taurocholate is limited solely by the cell membrane since this compound permeates the unstirred water layer more rapidly than the lipid cell membrane and stirring does not enhance uptake. With less polar bile acids which permeate the cell membrane relatively more rapidly, however, the unstirred water layer does exert resistance to mucosal uptake of these compounds. That the unstirred water layer is even more rate limiting to uptake from micellar solutions is indicated by the facts that the rate of bile acid absorption from such solutions is lower than from corresponding monomer solutions, stirring markedly enhances uptake from micellar solutions while increases in viscosity of the incubation media depress uptake and expansion of the micelle size further depresses absorption rates. We also have examined the important question of whether the micelle crosses the brush border intact once it reaches the aqueous-lipid interface. The observations that the calculated permeation rate of the micelle should be extremely low, the rate of mucosal cell uptake plateaus at a constant value when the critical micelle concentration is reached at the aqueous-lipid interface, and the different components of a mixed micelle are taken up at different rates indicate that uptake of the intact micelle does not occur; rather, bile acid absorption must be explained in terms of monomers in equilibrium with the micelle. Finally, after correction of the permeability coefficients of the various bile acids for the unstirred layer resistance the incremental partial molar free energy of solution of the hydroxyl group in the brush border membrane was calculated to equal -6126 cal.mole(-1) indicating that passive diffusion of these compounds occurs through a very polar region of the cell membrane.