Absorption of bile acids by the distal ileum is an essential component of the enterohepatic circulation. In neonatal rats, the appearance of the apical sodium-dependent bile acid transporter (ASBT) at 17 d of age coincides with increases in serum corticosterone and thyroxine. We tested the hypothesis that these hormones modulate ASBT expression during ileal development. Taurocholate uptake into the isolated ileum of normal 20-d-old pups exhibited saturable (K m ϭ 0.52 mM, J max ϭ 0.34 pmol mg/min) and nonsaturable (K diff ϭ 0.015 min Ϫ1 ) components and was two to five times greater than uptake in the proximal intestine. Hypothyroid or euthyroid pups received daily thyroxine injections starting at 6 d of age. At 12 d of age, serum concentrations of thyroxine, ileal abundance of ASBT mRNA, and ileal rates of taurocholate uptake were low in hypothyroid pups that received an injection of vehicle (HTϪ) or thyroxine (HTϩ) and in euthyroid pups that received an injection of vehicle (ETϪ) or thyroxine (ETϩ). At 20 and 26 d, ileal ASBT mRNA abundance and taurocholate uptake rate remained low in HTϪ pups but increased dramatically in ETϪ and ETϩ pups, paralleling the increase in serum thyroxine. Restoration of normal plasma thyroxine in HTϪ pups by thyroxine injections (HTϩ) restored normal ASBT development. Sodium-glucose co-transporter activity and mRNA expression were independent of serum thyroxine levels. Corticosterone levels were significantly lower in pups that were adrenalectomized at 10 d of age. ASBT mRNA abundance and taurocholate uptake rate increased markedly with age but were the same in adrenalectomized, sham-operated, and nonoperated pups. Hence, endogenous thyroxine but not corticosterone regulates the developmentally timed appearance of ASBT. Bile acids expelled from the gallbladder into the intestinal lumen emulsify dietary lipids to aid digestion, then form micelles with the products of lipid digestion to facilitate absorption. Synthesis of bile acids by the liver is insufficient to meet physiologic needs, and bile acids are recycled in a process that involves their absorption from the intestinal lumen back into the circulation and eventual resecretion from the liver. Bile acids are absorbed passively and in modest amounts in the jejunum (1) but are absorbed actively and in greater amounts in the distal ileum by the apical sodium-dependent transporter (ASBT) located at the brush border membrane of the enterocyte (2). It is not clear when active bile acid absorption begins during ontogenetic development of the small intestine. Sodium-dependent uptake of the bile acid taurocholate has been observed in the small intestine of an 8-mo-old child but not in that of a fetus or a neonate (3). Because reabsorption in the distal small intestine of neonates is inefficient or absent,~10 -20% of fat intake in the formula-fed newborn is thought to be malabsorbed, even though the rate of bile acid synthesis in the neonatal liver is markedly higher than that of older age groups. In rat pups, ASBT mRNA, protein, and ac...