Repeated subcutaneous injections of cholecystokinin octapeptide (CCK-OP) to Syrian golden hamsters for 10 days elicited a marked trophic effect on the pancreas, characterized by increased pancreatic weight and increased RNA/DNA ratios with an enhanced content of amylase in the pancreas in treated hamsters. These responses were observed after relatively small doses (100--300 ng CCK-OP x day-1 . 100 g body weight-1) of the hormone over a 10-day period. Although there was a small increase in total pancreatic DNA there was no definite evidence of hyperplasia in response to CCK-OP. DNA synthesis, as measured by histoautoradiography of tritiated thymidine labelled tissues, was increased in pancreatic acinar and islet cells, but not in ductal cells. This investigation, therefore, demonstrates a trophic action of CCK on the pancreas of the hamster similar to that reported for the rat and this may have relevance in studies of carcinogenesis since the hamster is an established species for studying pancreatic cancer.