The effects of a small dose of somatostatin (0.025 mg/h) on the pancreatic secretion of bicarbonate, amylase, and chymotrypsin during stepwise increasing doses of secretin was examined in six healthy volunteers. The secretion of bicarbonate, amylase, and chymotrypsin in response to secretin was significantly reduced by somatostatin. Both output and concentration of pancreatic enzymes were reduced, whereas the concentration of bicarbonate remained unchanged. The pattern of inhibition suggests that somatostatin is a competitive inhibitor of secretin in the stimulation of pancreatic secretion of bicarbonate, which supports the hypothesis of a direct effect of somatostatin on the exocrine secretory cells of the pancreas. The pattern of inhibition of amylase and chymotrypsin secretion is different and difficult to interpret from the present study, but somatostatin may inhibit also the secretin stimulated pancreatic secretion of enzymes completely, as the inhibitory effect seemed to decline when larger doses of secretin were applied.