The synergistic effects of dexamethasone (DEX) and thyroxine (T4) on the postnatal maturation of the 13-d-old rodent small intestine has been studied. Previous studies have shown that hydrocortisone and T4produced a synergistic response in enzyme maturation. However, T4elevates corticosteroid-binding globulin, which reduces the clearance of hydrocortisone. Thus, the apparent synergy between T4and hydrocortisone may have been due to increased glucocorticoid availability. DEX, which does not bind to corticosteroid-binding globulin, was given (d 8–12) at 25 pmol (i.e. 0.01 μg)/g body wt/d as established by a dose-response study in which this dose of DEX induced one third the maximum response in sucrase activity. In this way, synergy with T4(130 pmol/g body wt/d, i.e. 0.1 μg/g body wt/d, d 5–12) could still be observed. Glucoamylase, lactase, acid β-galactosidase, alkaline phosphatase, and sucrase activities were determined in two regions of the small intestine. Overall, the results for the two hormones administered alone showed intestinal maturation to be not significantly affected in the T4group and partially stimulated in the DEX group. When combined, DEX + T4synergistically increased jejunal sucrase, ileal glucoamylase, and duodenal alkaline phosphatase, and lowered ileal acid β-galactosidase. The striking exceptions to the general pattern were two brush border enzymes that normally decline during intestinal maturation, namely ileal alkaline phosphatase and jejunal and ileal lactase. For these enzymes, DEX alone did not elicit precocious maturation, and there was no evidence for a synergistic interaction of these two hormones. Serum corticosterone concentrations also were measured. When corticosterone concentrations were compared with enzyme activity, no correlation was found. Thus, DEX + T4act synergistically in the maturation of four enzymes in the rodent small intestine. Lactase and ileal alkaline phosphatase are regulated differently. Further studies are required to determine the mechanism through which T4enhances the DEX effect. (Pediatr Res 32: 306–311, 1992)