The influence of the cholesterol and protein diet on the mixed-function oxidases system activity and desaturation of fatty acids were examined. The phenobarbital innducibility of these parameters was analyzed too. Investigations were carried out on Wistar adult male rats. The animals were on the cholesterol (0.25%) and protein diet (32%) during 45 days. The cytochrome P-450 and cytochrome b5 contents, the NADPH-cytochrome P-450 reductase and NADH-cytochrome b5 reductase activities, aniline hydroxylase and 4-aminopyrine N-demethylase activities were measured in the hepatic microsomal fraction. In both sexes, the cholesterol diet decreased all the examined parameters with the exception of NADH-cytochrome b5 reductase activity. The protein diet did not change the examined enzyme activities or levels with the exception of induction of NADH-cytochrome b5 reductase and 4-aminopyrine N-demethylase activities. Simultaneous treatment with phenobarbital had heterogenous effect dependent on the type of diet and examined parameter.