Specific inhibitors acting upon pancreatic proteinases in the gut can cause pancreatic hypertrophy ('growth'), which is probably mediated through a feedback mechanism utilizing cholecystokinin. We have proposed the involvement of somatostatin, and here test the hypothesis that endogenous somatostatin secreted into pancreatic juice may regulate pancreatic growth. Groups of rats were given the proteinase inhibitor camostate intragastrically for either 3, 7, 14, 28, or 56 days, when they were sacrificed. In some groups the pancreata were weighed and homogenized while in other groups isolated perfused pancreatic organ preparations were performed. Somatostatin was measured in the homogenates, pancreatic juice and portal vein effluents. In camostate-fed animals, pancreatic weights increased to a maximum at 28 days, while pancreatic somatostatin content increased significantly from the third day onwards, and somatostatin secretion into pancreatic juice increased progressively until day 28. In contrast, somatostatin secretion into portal blood remained unchanged from those of untreated controls over the duration of the experiment, and its concentration was lower than in pancreatic juice. These observations provide further evidence that endogenous pancreatic somatostatin may control pancreatic growth in rats.