In this study, the chronic effects of proglumide (PGM, a cholecystokinin/gastrin receptor antagonist) on gallstone formation and hepatic bile secretion were investigated as follows: Group 1: Fed with low protein (14%) lithogenic diet. Group 2: Fed with the same lithogenic diet and was given PGM (250 mg/kg, bid, p. o.). Group 3: Fed with commercial guinea pig chow (protein content 22%). Eight weeks later the animals were operated under urethane anesthesia, the gallbladders were removed and examined for gallstones. Meanwhile, by bile duct cannulation, the hepatic bile flow and bile contents were measured. It was found that: (1) the animal model was valid for the purpose specified; (2) the rate of gallstone formation was significantly lower in PGM group than in the controls (17.5% vs 56.8%, P less than 0.01); and (3) PGM significantly enhanced the flow rates and electrolyte contents and decreased the unconjugated bilirubin content of the hepatic bile. It is concluded that PGM may suppress gallstone formation in guinea pigs on lithogenic diet, and this may be related to its stimulatory effect on hepatic bile secretion and to its ability to induce a decrease in unconjugated bilirubin in the hepatic bile.