The effects of methyl n-alkyl ketones and n-alkylbenzenes on hepatic cytochrome P450s in vivo and in vitro were investigated. Male rats were treated with acetone, methyl ethyl ketone, methyl n-propyl ketone, methyl n-butyl ketone, benzene, toluene, ethylbenzene, n-propylbenzene, or n-butylbenzene. The methyl n-alkyl ketones induced the metabolic activities of hepatic microsomes toward aminopyrine, 7-ethoxycoumarin, and aniline. n-Alkylbenzenes induced aminopyrine and 7-ethoxycoumarin metabolic activities. Testosterone 2 beta- and 6 beta-hydroxylation activities were induced by ketones with a long side chain such as methyl n-butyl ketone. Testosterone 2 alpha-hydroxylation activity was decreased by treatment with methyl n-butyl ketone. Testosterone 16 beta-hydroxylation activity was induced by treatment with methyl n-alkyl ketones. The inducibility was dependent on the length of the side chain. Testosterone 16 beta-hydroxylation activity also was induced by n-alkylbenzenes. These results indicate that the levels of multiple forms of cytochrome P450 were changed by treatment with these chemicals. P450IIE1, an acetone-inducible form, was induced by methyl n-alkyl ketones or n-alkylbenzenes. The inducibility did not depend on the length of the side chain of these chemicals. P450IIB1 and IIB2, both phenobarbital-inducible forms, were induced with methyl n-alkyl ketones and n-alkylbenzenes to an extent depending on the length of the side chain of these chemicals. Thus, the hydrophobicity of the inducer affected phenobarbital-type induction but not the induction of P450IIE1. We further investigated the interactions of ketone and benzene derivatives with cytochrome P450 in vitro. Testosterone hydroxylation activities of hepatic microsomes were measured in the presence of methyl n-alkyl ketones and n-alkylbenzenes. Methyl n-alkyl ketones inhibited testosterone 16 beta-hydroxylation activity. n-Alkylbenzenes inhibited 2 beta-, 6 beta-, 15 alpha-, 16 alpha-, and 16 beta-hydroxylation activities. Testosterone hydroxylation activities were inhibited by these chemicals depending on the length of the side chain. n-Alkylbenzenes were stronger inhibitors than methyl n-alkyl ketones, n-Butylbenzene was the strongest inhibitor of these activities. These results indicate that hydrophobicity was important in the interaction of these chemicals with cytochrome P450, and that there is some relationship between the inducibility of cytochrome P450 and its interaction with inducers.