The effect of insulin-dependent diabetes on the hepatic microsomal activity of cytochrome P450III and P450IV family proteins was investigated in rats pretreated with streptozotocin. In order to discern between the effects of the diabetogen per se and those of the ensuing diabetes, streptozotocin-treated rats received in addition either nicotinamide to prevent the onset of diabetes or daily treatment with insulin to antagonize the effects of diabetes. Streptozotocin-treated rats displayed higher ethylmorphine and erythromycin N-demethylase activities and lauric acid hydroxylase activity. Increases were also detected immunologically by using monospecific polyclonal antibodies against the P450III and P450IV families. All effects were prevented by nicotinamide and effectively antagonized by insulin. In order to evaluate the role of the ketone bodies in the diabetes-induced increases in the above activities, rats were rendered hyperketonaemic by dietary administration of medium-chain triacylglycerols. These hyperketonaemic animals displayed high laurate hydroxylase activity and P450IV apoprotein levels, similar to those seen in the diabetic animals. Hyperketonaemia induced by dietary means caused a modest increase in the demethylation of erythromycin and had no significant effect on the N-demethylation of ethylmorphine. Furthermore, no marked increases were evident in the P450III apoprotein levels in the hyperketonaemic animals. It is concluded that insulin-dependent diabetes induces proteins of the P450III and P450IV families, and that the hyperketonaemia that accompanies diabetes is largely responsible for the changes in the latter family.