To confirm the role of cholecystokinin (CCK) and secretin in digestion, exocrine pancreatic secretion, plasma CCK, and secretin were measured simultaneously in six dogs prepared with gastric and pancreatic fistulas after feeding a solid meal. Plasma CCK concentration determined by radioimmunoassay increased significantly from the basal level, reached a peak 35 min after meal ingestion, and after a dip it further increased toward the end of the 3-h observation. Pancreatic protein output increased significantly, peaked at the fifth 10-min period, and then declined progressively. Plasma CCK concentration and pancreatic protein output correlated significantly during the first postprandial hour. Plasma secretion demonstrated significant elevation at 15 min and a peak at 25 min after a meal. Plasma secretin correlated significantly with both bicarbonate output and flow rate during the 3 h. Simultaneous measurements of plasma CCK and secretin and of pancreatic secretion suggested that postprandial pancreatic secretion is primarily mediated by releases of CCK and secretin, but these hormones do not seem to be the only factors responsible for the secretion.