We investigated the effect of banol and paraoxon on the activity of NADPH-cytochrome c reductase and cytochrome P-450 concentration in microsomes of rats treated in vivo with these compounds alone, or in the presence of phenobarbitone. The results showed that banol reduced the NADPH-cytochrome c reductase activity and the concentration of cytochrome P-450, while paraoxon reduced the activity of NADPH-cytochrome c reductase, but increased the cytochrome P-450 concentration. Phenobarbitone stimulated an increased NADPH-cytochrome c reductase activity and cytochrome P-450 concentration. When applied with banol and paraoxon, phenobarbitone lowered their inhibitory effect.