Adult BALB/c mice, which are sensitive to hyperoxia (LT50 = 4.5 days 100% O2), were made tolerant to 100% O2 after treatment with butylated hydroxytoluene (BHT). Following a single ip dose of 400 mg/kg, mice survived longer periods in O2 when exposed to O2 at 7, 14, and 21, but not 2 days, following BHT injection. The tolerance was most pronounced on Day 7 (LT50 = 9.6 days) and decreased with time (LT50 7.7 days on Day 14 and 7.3 days on Day 21). Glucose-6-phosphate dehydrogenase levels of whole lung homogenates following BHT exposure were elevated on Day 7 when expressed as per milligram of protein or DNA. Other antioxidant defenses were generally increased only when expressed on a per lung basis. Histopathology of lungs from BHT-treated mice revealed typical BHT-induced lung lesions. BHT treatment followed by long-term hyperoxic exposure produced additional damage to the lung manifested by the exudative phase of diffuse alveolar damage with 1 week of exposure. This was followed by the proliferative phase, then chronic interstitial pneumonitis and fibrosis with 2 and 6 weeks of exposure, respectively. Mice continued to survive in 100% O2 despite this damage. We conclude that pretreatment with BHT enhances O2 tolerance in mice, which may be mediated by induction of antioxidant defenses and also by cell renewal induced by BHT damage.