In vivo studies using pentobarbital sodium-anesthesized dogs were performed to investigate whether a transport pathway for pyruvate and lactate is present in the exocrine pancreas. Concentrations of both acids were measured in peripheral blood and pancreatic juice before and after intravenous 15-min administration of 2.25 mmol/kg DL-lactate, superimposed on continuous intravenous infusion of 3 U X kg-1 X h-1 secretin. The concentration ratio of lactate to pyruvate in pancreatic juice was found to be approximately 1, a lower value than found in other tissues or body fluids. D-lactate, not detected in either blood or juice during basal periods, rapidly appeared in both fluids in parallel with the physiological isomer L-lactate, after the infusion of DL-lactate. Further addition of acetazolamide or a high dose of secretin caused a marked secretory response of pyruvate and lactate with no increase in juice bicarbonate levels. We conclude that these acids can be transported from blood to lumen when the transmembrane pH gradient across the duct cell membranes is augmented by stimulants via a proton pump mechanism involving a CO2-HCO3-buffer system.