Male LEW/f Mai rats were exposed to an atmosphere of 20 per cent N2O, 20 per cent O2, and 60 per cent N2 for a maximum of 35 days. Evidence of injury to the seminiferous tubules was found in some animals by the second day. By 14 days, such damage was found in all animals. The toxic effect was confined to the spermatogenic cells, with consequent reduction in mature spermatozoa and appearance of multinucleated forms. Other cells within the testes were resistant to damage. Recovery of spermatogenesis occurred after return to room air for more than three days. Serum testosterone levels were not significantly affected during the prolonged exposure.