Prior animal models of cholesterol gallstone formation have been criticized for their dissimilarity to the conditions of humans with gallstones. We present a new hamster model of cholesterol cholelithiasis that more closely approximates the human situation. Sixty female Golden Syrian hamsters (average weight 83.2 +/- 3.4 g) were allocated to six groups of 10 animals each. Groups were fed standard diet (containing 0.8 gm cholesterol/g of food) or increased cholesterol diet (containing 2.4 mg cholesterol/g of food), with or without ethinyl estradiol, 15 micrograms/kg/d. Two groups receiving both increased cholesterol and ethinyl estradiol also received either chenodeoxycholic acid or ursodeoxycholic acid, 20 mg/kg/d. The animsl were sacrificed at 12 wk. Cholesterol gallstones (78.3 +/- 5.0% cholesterol by weight) formed in 30% of the animals fed ethinyl estradiol, 50% of those fed increased cholesterol, and 90% of those fed the combination of both. Bile was saturated in all three groups, with the saturation index of the combination group (2.08 +/- 0.17) being the highest. In both groups receiving bile acid therapy, no gallstones were found, and the bile remained unsaturated. For the bile acid-fed groups, both hepatic HMG-CoAR and hepatic cholesterol 7 alpha-hydroxylase activities were reduced (P less than 0.01) when compared to the group fed standard diet and to the grou fed the combination. Thus, a new animal model of cholesterol gallstone formation has been developed in which chenodeoxycholic acid and ursodeoxycholic acid therapy prevented gallstone formation through mechanisms similar to those reported in cholesterol gallstone patients.