In the present study pancreatic secretion and plasma cholecystokinin (CCK) levels were analyzed in eight volunteers after daily ingestion of the serine protease inhibitor camostate for 5 days. This was compared with the effect of a single intraduodenal dose of camostate. Prolonged administration of camostate for 5 days had no effect on basal and stimulated pancreatic secretion and plasma CCK. A single dose of camostate completely inhibited enzymatic activity of trypsin and chymotrypsin and stimulated volume, amylase, and lipase secretion but induced an only slight and insignificant increase in plasma CCK. After the ingestion of a test meal, camostate did not influence stimulated enzyme secretion and increased plasma CCK. We concluded that the intraduodenal perfusion of camostate stimulated pancreatic secretion by a feedback mechanism that is not mediated by CCK. The repeated oral administration of camostate did not induce adaptive changes in pancreatic secretion.