Various kinds of miso (paste and freeze-dried, low salt-dried and desalt-dried powders; 20% in basal diet) with or without benzo (a)pyrene (B(a)P; 1, 000ppm in basal diet) in the diet were given to male rats for a period of 14 days, and changes in hepatic drug-metabolizing enzymes (cytochrome P-450, aniline hydroxylase, aminopyrine N-demethylase and glutathione S-transferase) were investigated. Each miso diet without B(a)P tended to increase all of the parameters studied. Furthermore, the miso diets with B(a)P generally elevated even the B(a)P-induced activities of drug-metabolizing enzymes. These data do not preclude the possibility that miso may have defensive action against carcinogenic chemicals by enhancing the activity of detoxification systems.