We examined whether endogenous cholecystokinin (CCK) is involved in growth of the preweanling rat pancreas. Twice daily for 14 days, 7-day-old neonatal rats received an oral gavage of either 2.5 mg/kg or 5.0 mg/kg of the potent and specific CCK receptor antagonist L-364,718 (the 2.5 mg/kg dose of antagonist was shown in the present study to abolish totally the pancreas growth-promoting effects of exogenously administered caerulein (CR) in neonatal rats). Control pups received oral gavages of the L-364,718 vehicle alone. The final body weights, pancreas weights, total pancreatic DNA contents, and total pancreatic protein contents did not differ significantly between the 21-day-old control pups and the 21-day-old pups that were pretreated for 14 days with either the low or the high doses of L-364,718. These findings suggest that endogenous CCK is not required for growth of the neonatal rat pancreas.