The effects of selenium supplementation on induction of cholangiocarcinomas and related precancerous lesions in female Syrian Golden hamsters by N'-nitrosobis(2-oxopropyl)amine (BOP) were investigated. Four-week-old animals were divided into two groups according to the selenium level contained in the drinking water (0.1 ppm or 4.0 ppm) and fed a purified diet containing less than 0.05 ppm of the trace element. Starting at Week 4 of the experiment, hamsters were administered 10 weekly injections of BOP (10 mg/kg body wt) and then killed 18 weeks after the last carcinogen administration. Animals receiving physiological saline alone served as controls. Cholangiocellular carcinomas tended to be reduced, and putative preneoplastic lesions of cholangiofibrosis were significantly decreased in the high-as opposed to the low-selenium groups in terms of both incidence rate and number per effective animal. The respective high and low selenium values for incidence and number were 24/38% and 0.34/0.66, respectively, for cholangiocarcinomas and 50/89% and 1.21/8.44, respectively, for cholangiofibroses. Proliferation of intrahepatic bile ducts was also significantly inhibited in the high-selenium group along with cyst formation. Biochemical investigation revealed both selenium level and glutathione peroxidase activity to be significantly greater in the high-than in the low-selenium group livers. The results thus suggest that selenium may inhibit BOP-induction of bile duct lesions, possibly via glutathione peroxidase-mediated alteration of carcinogenesis.