The possibility that cholesterol 7 alpha-hydroxylase activity and cholesterol metabolism could be mediated by chemical inducers of the microsomal mixed function oxidase (MFO) system was investigated. Groups of male Wistar rats weighing about 250 gm were treated with the inducer compounds, DDT, phenobarbital, or 3-methylcholanthrene, or with the hypocholesterolemic drugs, D-thyroxine, clofibrate, or with pharmacological doses of nicotinic acid. Although clofibrate and nicotinic acid produced some increases in MFO activity as reflected in p-nitroanisole-o-demethylase and aniline hydroxylase, no change in the activity of cholesterol 7 alpha-hydroxylase was produced by the hypocholesterolemic drugs. The inducer compounds produced typical responses of increased liver weight, microsomal protein, and p-nitroanisole-o-demethylase, and aniline hydroxylase activities. 3-Methylcholanthrene and DDT, however, both caused a significant decrease in cholesterol 7 alpha-hydroxylase activity. 3-Methylcholanthrene, which produced the greatest decrease in cholesterol 7 alpha-hydroxylase activity, also caused a marked elevation in serum cholesterol levels. It seems plausible that the decrease in cholesterol 7 alpha-hydroxylase produced by 3-methylcholanthrene may have produced the elevation in serum cholesterol levels.