The metabolism of benzo[a]pyrene in reconstituted pulmonary mono-oxygenase systems has been studied. Metabolites formed by pulmonary cytochrome P450MC, a major form of pulmonary cytochrome P-450 isolated from 3-methylcholanthrene-treated rats, were analysed by h.p.l.c. The profiles of benzo[a]pyrene metabolites formed by the reconstituted P-450MC systems were different from that obtained with rat-lung microsomes, indicating the presence of several unknown metabolites in the reconstituted systems containing NADPH-cytochrome P-450 reductase and epoxide hydrolase. 3-Hydroxybenzo[a]pyrene was a major product formed by pulmonary cytochrome P-450MC, in the absence or presence of epoxide hydrolase. The addition of purified epoxide hydrolase to the reconstituted systems increased the formation of dihydrodihydroxy-benzo[a]pyrenes, particularly 7,8-dihydro-7,8-dihydroxybenzo[a]pyrene. The 9,10-dihydro-9,10-dihydroxybenzo[a]pyrene was the major dihydrodiol formed by pulmonary cytochrome P-450MC. By the addition of epoxide hydrolase the total amount of phenols decreased in parallel with an increased production of dihydrodiol, but the amount of quinones was not changed. Similar results concerning the related production of phenols and dihydrodiols, in the absence and presence of epoxide hydrolase, were obtained in reconstituted systems of hepatic cytochrome P-450MC, the major form of hepatic cytochrome P-450 from 3-methylcholanthrene-treated rats.