1. The inhibitory effects, of a number of compounds, on the formation of two groups of benzo(a)pyrene metabolites in rabbit lung homogenates (9000 g supernatant) have been investigated. 2. The inhibitors comprised two groups: those inhibiting primarily benzo(a)-pyrene hydroxylase activity, and those inhibiting the activity of the epoxide-metabolizing enzymes as well as hydroxylase activity. Phenol, 1-naphthol, nicotine and acetone belong to the former group; naphthalene, anthracene and chlorpromazine to the latter group. 3. The most active inhibitors also caused a significant decrease in the amount of benzo(a)pyrene bound covalently to tissue macromolecules.