At low oral dosages (15 μmole/kg) neither mirex nor 3,4,5,3′,4′,5′-hexachlorobiphenyl was found to significantly elevate hepatic microsomal cytochrome(s) P-450 in the mouse, 7 days post-treatment. The associated drug metabolic activities, rmaminopyrine- N-demethylase, p-nitroanisole- O-demethylase, and aniline hydroxylase, were also not significantly elevated above control levels. However, aminopyrine- N-demethylase and p-nitroanisole- O-demethylase activities were significantly elevated by a 1:1 molar combination of the two compounds (“Both”), administered at a total molar concentration (15 μmole/kg) equal to that of either of the two constituents. Concomitant administration of the molar sum of Mirex and the hexachlorobiphenyl significantly elevated all three metabolic activities and cytochrome(s) P-450 concentration. The relatively greater effect of “Both” in elevating these activities may be due to the simultaneous elevation of cytochromes P-450 and P-448.