Wild type populations of Drosophila melanogaster were exposed to 21%, 33%, 45%, and 55% oxygen at normal atmospheric pressures. Individuals were selected randomly at 5-d intervals and inspected for vacuolation of the forebrain. Longevity of the populations was also measured. Flies in 21% and 33% oxygen had identical survivorship and even after 50 d showed little vacuolation. Flies maintained in 45% oxygen showed rapid mortality between 30-40 d and were found to have vacuolated forebrains. Populations treated in 55% oxygen declined rapidly in numbers between 20-30 d and showed severe vacuolation of the forebrain. It was considered important that normal longevity and normal aging of brain cells in 33% oxygen at standard pressure were found because similar partial pressures of pure oxygen were reported to significantly reduce longevity. It is suggested that, because insects are known to be tolerant to fairly extreme changes in total atmospheric pressure, a partial pressure of nitrogen as a diluent gas is the important factor in the toxic effects of a given pressure of oxygen. The study indicates that in a nitrogen/oxygen mix at 760 torr total pressure, the threshold to toxicity lies near, or just beyond, 300 torr oxygen. The authors indicate that preliminary findings in their current work show that if a small partial pressure of nitrogen is maintained, the total pressure can be reduced considerably without affecting this threshold level.