To investigate the effect of hyperbaric oxygenation on superoxide dismutase activity, neonatal rats (7-10 days old) and adult rats (approximately 100 days old) were continuously exposed to hyperbaric oxygen environment of 2.4ATA for 8 hours and their superoxide dismutase activity were measured. Neonatal rats, all survived through exposure, showed significant increases in the pulmonary superoxide dismutase activity at immediately and 24 hours after exposure. Adult rats, whose 8 hour survival rates were 14%, did not show any significant increase in the activity of pulmonary superoxide dismutase as compared to the control adult rats. These findings are indicating that increased tolerance to oxygen toxicity in neonatal animals during exposure may be attributed to the increase in activity of superoxide dismutase in neonatal rats.