Repeated injections of cholecystokinin (CCK) during a period of up to 3 weeks significantly increased the weight of the pancreas in rats. This was associated with an increase in the amount of protein per unit weight of DNA, suggesting hypertrophy of the acinar cells, and with increase in the total amount of pancreatic DNA, indicating additional hyperplasia of the gland. CCK administration also increased the pancreatic content of amylase and trypsin, but the content of lipase remained unchanged. The rate of secretion of the two enzymes increased in the CCK-treated rats, although it appeared that the functional capacity of the individual pancreatic acinar cells was not increased. CCK injections had no effect on the insulin content of the pancreas or on the composition of the partoid glands. Repeated injections of secretin in the doses used in this study had no effect on the pancreas.