Exposure of Drosophila melanogaster adult males to an atmosphere of O2-N2 (1:1) at 760 Torr resulted in a marked reduction of life span. Among the fine structural changes occurring in the oxygen-poisoned flies, the following were similar to age-related changes: ceroid-lipofuscin accumulation in the visceral organs, sponginess of the brain and accumulation of lamellated dense bodies in the nerve cells. Also, the oxygen-poisoned flies showed a loss of the cytoplasmic membranes of the sperm cells, an alteration which was previously found in y-irradiated, but not in normal senescent, flies. In our opinion, oxygen poisoning of Drosophila may provide a valid model of accelerated aging, at least in regard to peroxidation injury of cell organelles and ceroid-lipofuscin accumulation.