In vitro uptake of taurocholate by weanling rat ileum was characterized. The neonatal development of the ileal mechanism was determined, and the effect of steroid treatment on this development was assessed. Mucosal uptake of taurocholate by ileum of 26-day-old and adult rats was linear with time and, when measured as a function of concentration, tended toward a plateau. Mucosal concentration of taurocholate was significantly reduced by metabolic inhibitors and was competitively inhibited by taurochenodeoxycholate. The capacity to concentrate taurocholate in ileal mucosa was undemonstrable in 12-day-old neonates, first appeared at 15 days, and reached adult levels by 26 days of age. Mucosal uptake of taurocholate by ileum of 12-day-old offspring of steroid-treated mothers or of 12-day-old neonates given dexamethasone directly was significantly increased over control values. The results indicate that the ileal bile salt absorption found in weanling rats is an active process that does not begin to develop in neonatal rats until 15 days after birth, and that the appearance of ileal active transport can be accelerated by maternal or neonatal steroid treatment.