The role of oral corticosterone (C) in the maturation of the neonatal rat jejunal barrier to the absorption of nonspecific macromolecules was evaluated. This was done by adding C to the diet of rat pups weaned at an early age, 17 d, from maternal milk (MM) to either a protein hydrolysate (PH) or soy (S) artificial formula. Both PH and S are known to cause a delay in small intestinal closure to the absorption of a 40-kD glycoprotein tracer, horseradish peroxidase (HRP), on d 21 of age. C was added to PH and S formulas from d 17 to 21 at 0.26 μmol/L (10 μg/dL), a level found in the MM of lactating rat dams, or at 10.29 μmol/L (400 Mg/dL) (PH + 10C, PH + 400C) (S + 10C, S + 400C). Controls consisted of rat pups fed PH or S without C and animals remaining with the dam on MM. The delay in jejunal closure to HRP on d 21 in both PH- and S-fed pups was prevented by C supplementation at both the higher and lower concentrations. Geometric mean (95% confidence intervals) jejunal HRP absorption in PH + 10C pups was 74 (32,167) IU HRP/mL ± cm ± min, less than in pups fed PH without C [353 (200,615); p <0.05] and indistinguishable from HRP absorption in MM-fed animals [111 (79,154)]. HRP absorption in PH + 400C pups [52 (23, 115)] was also less than that in animals fed PH without C (p < 0.01) and indistinguishable from those fed MM. In S-fed pups, closure delay was accompanied by a lamina propria eosinophilia not seen with PH or MM feedings; this did not occur in S + 400C pups. Our results lend support to the view that C, a glucocorticoid known to be present in rat MM, may play a role in the normal ontogenetic closure of the small intestine to macromolecular absorption in the neonatal rat.