Inhibition of pulmonary CYP4B1 activity by pretreatment of rats with p-xylene decreased the ability of lung microsomes to N-hydroxylate 2-aminofluorene and prevented the lung damage normally seen after dosing with ipomeanol. The toxicity of ipomeanol, as assessed by acute lethality, was decreased by a factor of eight. In contrast, induction of CYP1A1 by Aroclor or beta-naphthoflavone, or inhibition of CYP2B1 by O,O,S-trimethyl-phosphorodithioate, as assessed by measurement of lung microsomal dealkylation of ethoxyresorufin or pentoxyresorufin, did not change ipomeanol toxicity. A polyclonal antibody raised against CYP4B1 prevented the covalent binding of [14C]-ipomeanol to lung microsomal protein in vitro. Antibodies raised against the other major P450 isozymes of rat lung, CYP2B1 and CYP1A1, had no effect on this binding. Aroclor, beta-naphthoflavone, and O,O,S-trimethylphosphorodithioate failed to affect binding of radiolabeled ipomeanol in vivo, but pretreatment with p-xylene resulted in a significant reduction in this binding. The CYP4B1 substrate 2-aminofluorene, when dosed to rats, caused a sixfold decrease in ipomeanol toxicity. These results indicate that in the rat, unlike the rabbit, pulmonary bioactivation of ipomeanol is predominantly dependent upon CYP4B1.