In male rats, genetic obesity and experimental diabetes are associated with altered activities of several of the hepatic microsomal P-450 isozymes concerned with steroid and xenobiotic oxidation. The present study examined the roles of insulin and ketonaemia in effecting these changes. In obese male Zucker rats, androstenedione 6 beta-, 16 alpha- and 16 beta-hydroxylase activities (mediated by P450PCN-E, P-450UT-A and P450PB-B, respectively) were significantly decreased to 21%, 20% and 43% of lean control. Obesity was also associated with a significant decrease in the activities of N-nitrosodimethylamine demethylase (P-450j) and aniline p-hydroxylase to about 70%. A similar decrease in total microsomal P-450 was also observed. Androstenedione 7 alpha-hydroxylase activity (mediated by P-450UT-F) was unchanged in these animals. In streptozotocin-induced diabetic male Wistar rats, androstenedione 7 alpha- and 16 beta-hydroxylase activities were significantly elevated to 230% and 270% of control, respectively. Significant increases in the rates of N-nitrosodimethylamine demethylase and aniline p-hydroxylase were also noted in diabetic rat liver. In contrast, the activity of P-450UT-A was reduced to 30% of control and P-450PCN-E-specific 6 beta-hydroxylation was unchanged. Control of the diabetic state with insulin treatment reversed all the changes in P-450-mediated activities. Significant correlations were found between serum concentrations of insulin and catalytic activities of P-450PB-B (rho = -0.46), P-450UT-F (rho = -0.65) and P-450j (rho = -0.71). Positive correlations of the same magnitude were also found between these mixed function oxidase activities and beta-hydroxybutyrate.(ABSTRACT TRUNCATED AT 250 WORDS)