Corticosterone was investigated for its ability to inhibit benzo(a)pyrene (BaP) metabolism and was compared to metyrapone and alpha-naphthoflavone. Corticosterone inhibited aryl hydrocarbon hydroxylase (AHH) activity nonlinearly in hepatic microsomes from uninduced, phenobarbital-induced, or 3-methylcholanthrene-induced rats. When compared to the classic inhibitors metyrapone and alpha-naphthoflavone, corticosterone had inhibitory properties similar to each. Metabolism of BaP to dihydrodiols was inhibited at the K-region by corticosterone only in uninduced microsomes. Dihydrodiol formation at the Bay region, which leads to the putative ultimate carcinogen, was not affected by corticosterone in uninduced or phenobarbital-induced microsomes but stimulated in 3-methylcholanthrene-induced microsomes. These findings suggest that corticosterone regioselectively inhibits cytochrome P-450 mediated oxidation of BaP to less mutagenic metabolites while stimulating the formation of highly mutagenic products.