The immediate postnatal period is a time of rapid pancreatic growth and development. Gastrointestinal regulatory peptides have been shown to exert trophic effects on the pancreas, and one such peptide, bombesin, has been shown to exert trophic effects on the pancreas of both the suckling and adult rat. Our previous studies had suggested that the sensitivity of the rat pancreas to bombesin might be changing during the suckling period. We therefore conducted experiments to determine if age-related changes in the responsiveness of rat pancreas to chronic administration of exogenous bombesin occur, and to characterize these changes. Beginning on day 3 ("suckling") or 24 ("weanling") postnatally, groups of rats were injected subcutaneously with several doses of bombesin tetradecapeptide every 12 h for 7 days. Bombesin injection produced a dose-dependent increase in pancreatic protein content and trypsin activity in both suckling and weanling groups, although the degree of increase was greater in the weanling group than in the sucklings. Significant increases in pancreatic contents of DNA and amylase activity were observed only in the weanling group. We have thus demonstrated for the first time that the pancreas of the suckling rat exhibits a diminished response to chronic parenteral administration of bombesin in terms of changes in protein and DNA contents, as well as enzyme composition, compared to the pancreas of weanlings. The elucidation of the physiologic basis for these differences may provide specific information regarding the mechanism of bombesin-stimulated pancreatic growth, as well as general information concerning the control of pancreatic growth during development.