Intraperitoneal administration of single doses of the volatile aromatic hydrocarbon, naphthalene, resulted in dose-dependent bronchiolar epithelial cell necrosis in mice. Twenty-four hours after a dose of 50 mg/kg, swelling of Clara cells with some exfoliation of epithelial cells was evident in half of the treated animals. At doses of 100 mg/kg small numbers of necrotic and swollen cells with pyknotic nuclei were observed. At 200 mg/kg there were substantial numbers of bronchiolar epithelial cells sloughed into the airway lumen, apical projections were virtually absent, and there were large numbers of cells with pyknotic nuclei. In contrast, bronchiolar airways from mice treated with naphthalene daily for 7 days at doses of 50, 100, or 200 mg/kg/day differed only slightly from controls. Significant protection to bronchiolar epithelial cell necrosis produced by 300 mg/kg naphthalene was afforded by seven daily injections of 200 but not 50 or 100 mg/kg naphthalene. A gradual recovery in sensitivity to the 300 mg/kg challenge dose of naphthalene was observed as the time between the last 200 mg/kg naphthalene dose increased from 24 to 144 hr. Daily administration of 200 mg/kg but not 50 or 100 mg/kg naphthalene for 7 days resulted in a selective decrease in the rate of formation of 1R,2S-naphthalene oxide by mouse lung but not liver microsomal enzymes. This selective decrease in pulmonary microsomal formation of 1R,2S-oxide continued in animals killed 48, 96, and 144 hr after the last administration of 200 mg/kg. Alterations in the rate of formation of reactive, covalently bound naphthalene metabolites in lung microsomes were not observed, nor were there any differences in the levels of covalently bound reactive metabolites in vivo between tolerant and control animals. These studies are consistent with other work showing that the lung loses susceptibility to the acute injury arising from repeated exposure to pneumotoxicants. In contrast to other studies with naphthalene where alterations in the levels of covalently bound reactive metabolites in the lung closely paralleled the extent and severity of bronchiolar injury, these studies clearly separate necrosis from covalent binding. Although the correlation was not absolute, it appears that formation of 1R,2S-oxide by microsomal enzymes in vitro is a better overall marker of decreased sensitivity to naphthalene-induced bronchiolar necrosis than is reactive metabolite binding either in vivo or in vitro.