Specimens of Oregon-R Drosophila melanogaster (Meigen) were bred in a defined sexual ratio in test chambers under different partial pressures of O2. In the gas mixture of 5--7% O2 plus 93--95% N2, the number of ovules dropped to nearly half even in the first generation, the time period of each development stage was considerably extended, and the size of the larvae decreased to a fraction of that of the controls. In the groups kept in a mixture of 10--12% O2 plus 88--90% N2, the same changes took place at a much slower rate, through several generations.