The postnatal development of intestinal bile salt transport was examined in the guinea pig. Using an in vitro technique, mucosal uptake of taurocholate was measured in jejunum and ileum at five monomeric concentrations from fetal, 1-day-old, 5-day-old, 10-day-old, 21-day-old, and adult animals. Jejunal taurocholate uptake was linear with respect to concentration and was not inhibited by taurochenodeoxycholate at all ages studied. In fetal and 1-day-old animals, ileal taurocholate uptake was linear and was not inhibited by taurochenodeoxycholate. A curvilinear relation between uptake and taurocholate concentration, which was inhibited by taurochenodeoxycholate, was observed in 5-day-old and older animals. These findings indicate that, in the guinea pig, passive diffusion of taurocholate is the only mode of intestinal transport in fetuses and newborns and the sole mode of jejunal transport at all ages. By 5 days, however, active ileal transport appears, which persists into adulthood and contributes to the development of an efficient enterohepatic circulation of bile salts.