In this study the inhibitory effect of glucagon on pure exocrine pancreatic secretion obtained by direct drainage of the duct of Wirsung in man is demonstrated. In two experiments carried out during stimulation with secretin and CCK-PZ and in one experiment during secretin stimulation an inhibitory effect on pancreatic secretion was found. This effect was global, affecting both the total volume and the concentration and output of protein and the output of bicarbonate. The greater inhibition, however, occurred in the case of protein.