To test the postulate that increased activity of the glutathione peroxidase system is required for the increased tolerance to oxygen toxicity that develops after several days of prior exposure to 85% oxygen we searched for a combination of increased tolerance but normal activity of the glutathione system. We exposed rats to 85% oxygen for 7 days and then placed them in air. After 0, 10, 20, and 30 days we estimated the potential role of an antioxidant system by measuring activities of glucose-6-phosphate dehydrogenase (G6PD), glutathione reductase (GR), glutathione peroxidase (GP), and the nonprotein sulfhydryl content (NPSH) of lung tissue. After 7 days in 85% oxygen (0 days in air) activities of G6PD, GR, and GP, were elevated above control values by 189%, 32%, and 126%, respectively, and NPSH was 146% higher. Twenty days later these activities and NPSH were not significantly different from those of control animals never exposed to 85% oxygen. We also tested these rats without increased enzyme activities for oxygen "tolerance" by exposing them, after 20 days of recovery in air, to 100% oxygen for 3 days and found that some were "tolerant" as judged by a mortality rate of only 42% compared with 100% in a group not previously exposed to oxygen. To determine if this degree of tolerance could be related to accelerated increases of enzyme activities during the exposures we measured the enzyme activities and NPSH of lungs at 12, 24, 36, 48 and 60 hr after the start of exposure to 100% oxygen in two groups: one preexposed to 85% oxygen 20 days earlier and the other not previously exposed to oxygen.(ABSTRACT TRUNCATED AT 250 WORDS)