Ileal bile acid binding protein (IBABP) is a cytosolic protein believed to be involved in the absorption of conjugated bile acids. In rodents this protein and its mRNA have been shown to increase markedly during the third postnatal week. Because this period of ontogeny is characterized by increasing circulating concentrations of glucocorticoids and thyroxine, the goal of our study was to investigate the role of these hormones in IBABP expression in the developing rat. Administration of various doses of dexamethasone (Dex) during the second postnatal week caused a robust induction of IBABP mRNA and protein. Plateau levels of IBABP mRNA occurred at a Dex dose of 0.1 microg/g body wt, which is within the physiological range. IBABP mRNA was not appreciably induced until 24 h after treatment, suggesting that glucocorticoids influence IBABP either through a delayed primary or a secondary response mechanism. The regional pattern of IBABP mRNA elicited by Dex mimicked that seen during normal development, with appearance in distal ileum preceding proximal ileum. Thyroxine injections did not result in a significant increase of IBABP mRNA, and synergism between Dex and thyroxine was not observed. Taken together, our data suggest that maturation of IBABP expression is influenced by glucocorticoids but not by thyroxine.