The long term administration of chenodeoxycholic acid in man has to be regarded with caution because chenodeoxycholic acid has caused liver damage in various species of animals, including primates. To study the effect of three doses of chenodeoxycholic acid (10, 40, and 100 mg per kg per day) on hepatic function and morphology, biliary bile acid composition and the reversibility of changes were investigated in 22 rhesus monkeys. After 6 months of treatment with 40 and 100 mg per kg per day, bile duct proliferation, portal tract inflammation and fibrosis, bile canalicular bleb formation, and hypertrophy of the smooth endoplasmic reticulum were associated with elevated serum levels of oxaloacetic transaminase, glutamic pyruvic transaminase, and leucine aminopeptidase. In the bile, the proportion of chenodeoxycholic acid and its bacterial metabolite, lithocholic acid, rose to approximately 85 and 10% of the total bile acids. After chenodeoxycholic acid was withdrawn for 3 months, the hepatic morphological lesions persisted in some animals although biliary bile acid composition returned to normal. No hepatic abnormalities were seen in the animals treated with 10 mg per kg per day. The findings suggest that long term treatment of rhesus monkeys with high doses of chenodeoxycholic acid results in severe hepatic histological lesions that can persist after discontinuation of the bile acid.