Although exogenous administration of cholecystokinin (CCK) or dietary manipulation to increase circulating CCK have previously been shown to promote pancreatic growth, the role of CCK in controlling normal pancreatic development remains unclear. A potent CCK receptor antagonist, L364,718, was administered to rats, guinea pigs, and hamsters to block the effect of endogenous CCK. Animals were given continuous infusions of L364,718 (25 nmol.kg-1.h-1), CCK octapeptide [(CCK-8) 200 pmol.kg-1.h-1], or both CCK-8 and L364,718 for 14 and 28 days. Adult (4-mo-old) and young (4-wk-old) animals were used. CCK-8 and L364,718 were administered via separate, subcutaneously implanted mini-osmotic pumps. Infusions of CCK-8 alone for 28 days resulted in a 21.7% increase in wet pancreatic weight in 4-wk-old rats and a 22.7% increase in 4-wk-old guinea pigs (both P less than 0.001 compared with controls). Similar increases were found in DNA, RNA, and total protein contents. Coadministration of L364,718 totally blocked the trophic effects of exogenously infused CCK-8 in rats and guinea pigs. Administration of L364,718 alone in hamsters, guinea pigs, and rats for 14 and 28 days failed to alter the normal growth of the pancreas gland as measured by these parameters. Although elevated levels of CCK appear to promote a potent trophic response in the growing pancreas, this regulatory peptide does not appear to be an essential trophic factor for the normal growth of the exocrine pancreas in these animals.