Seventeen-day-old chick embryos were used as a test system to assess the effect of vitamin K1(K1) on benzo(a)pyrene (BP) metabolism as measured by the induction of arylhydrocarbon hydroxylase (AHH) and cytochrome P-450 and the levels of glutathione (GSH) and glutathione S-transferase (GST) in liver. Twenty-four hours after injection of BP into the air sac there was a sharp rise in AHH and P-450 and a drop in GSH. When K1 was injected 24 hr prior to BP there was a decrease in GST activity as compared with the control plus an augmented increase in AHH induction. This augmentation in BP metabolism (Phase I) together with a concomitant decrease in at least one mechanism of Phase II conjugation is in keeping with other evidence that K1 can play an adjuvant role in BP induced mutagenicity and carcinogenicity. Ubiquinone has a much lesser effect on BP metabolism than does K1 in equimolar concentration.