Ferric citrate was examined for its possible induction of acute and chronic changes in bile acids in rats. Three hours after a single intravenous Fe(III) injection both the biliary bile acid level and the percentage of cholic acid in bile were significantly higher in treated rats than in control rats. However, no significant difference was observed after treatment with the same daily dose for one week. The superoxide dismutase (SOD) activity in liver tissue increased in the chronic group. These results suggest that oxygen free radicals, mainly the hydroxyl radical, promote hydroxylation during bile acid biosynthesis by enhanced monooxygenase activity.