The reactivities of four alkoxyphenoxazone derivatives toward cytochrome P450-dependent monooxygenase activity were studied in lung microsomes from control and beta-naphthoflavone (BNF)-treated rats. Ethoxyphenoxazone (EtOPh)- and methoxyphenoxazone (MeOPh)-dealkylases were induced by beta NF (16- and 3.9-fold respectively), whereas benzyloxyphenoxazone (BzOPh)- and pentoxyphenoxazone (PeOPh)-dealkylase activities were unchanged after pre-treatment with beta NF. The specific activities of BzOPh'ase from control- and beta NF-treated rats and EtOPh'ase from beta NF-treated rats were sufficiently high (214-560 pmoles resorufin per min per mg protein) for routine biochemical studies. Each differed in its sensitivity toward the in vitro inhibitors, alpha-naphthoflavone (ANF) and metyrapone (MP). In lung microsomes from beta NF-treated rats the I50 (MP) = 7.6 X 10(-5) M for EtOPh'ase and 1.0 X 10(-6) M for BzOPh'ase. I50 (ANF) = 3.8 X 10(-8) M for EtOPh'ase. I50 (ANF) for BzOPh'ase could not be determined; at the highest concentration of ANF (10(-3) M), 50% inhibition was not observed. The presence of high levels of two distinct forms of lung microsomal P450-dependent alkoxyphenoxazone dealkylases, BzOPh'ase from untreated and beta NF-treated rats and EtOPh'ase from beta NF-treated rats, will be useful for studies on specific P450-xenobiotic interactions in rat pulmonary tissue.