Nine anesthetized dogs were provided with acute common duct fistulas after exclusion of the gallbladder. Synthetic Substance P was administered as caval infusions in a dosage of 0.5-20 ng x kg-1 x min-1, duration 10 min. The output of hepatic bile, sodium and amylase decreased during infusion by 40-52 per cent at the highest doses. After termination of infusion all 3 parameters increased by 19-60 per cent above the basal level. The biliary concentration of sodium was constant, while that of amylase increased during infusion. The responses were dose-related. The anticholeresis induced by substance P might be due to inhibition of the canalicular bile fraction, which presumably is mediated by active sodium transport and independent of bile salt excretion.