The action of somatostatin on compostition and flow rate of pure pancreatic juice obtained by endoscopic cannulation of the main pancreatic duct was evaluated in 5 healthy volunteers. Synthetic secretin (0.06 CU/kg-h) was intravenously infused throughout the 80-min study. Bicarbonate concentrations in pancreatic juice achieved constant levels (117 +/- 3 muEq/ml) after 10 min, whereas a steady state of juice flow (7.3 +/- 1.4 ml/5 min) was attained after 15 min of secretin infusion. In the third 20-min period, cyclic somatostatic (5 mug/kg-h i.v.) was given, leading to a decrease in pancreatic flow rate by 47% after 10 min, and by 67% after 15 min of somatostatin administration. Alrady 5 min after the infusion of somatostatin had been discontinued, pancreatic flow rate gradually recovered; presomatostatin levels, however, were not reached within 20 min. Cyclic AMP varied roughly in accordance with bicarbonate concentrations, whereas the chloride concentrations were reciprocally related. Bicarbonate, sodium, potassium, protein, and cyclic GMP concentrations did not change substantially due to somatostatin.