A B ST R ACT Fruit flies live longer at the partial pressure of oxygen found in air than at either larger or smaller partial pressures. Flies exposed to 1 atm of oxygen for 8 hr every day do not recover completely in the remaining 16 hr. In general, intermittent exposures to 1 atm of oxygen are better tolerated than continuous exposure to the same average oxygen concentration per day, but exposures to higher pressures of 2-5 atm of oxygen for as little as a half hour every two days markedly shorten the life-span. Older flies consume more oxygen per minute and are more sensitive to oxygen poisoning than young flies, and the rate of dying in 6 atm of 02, or the reciprocal of the survival time, is a linear function of the age. The oxygen pressure-time curve can be well expressed by the general empirical equation (Po2) ~ X time = 120 where P is in atmosphere and survival time in hours. The progress of oxygen poisoning appears to be linear with time rather than exponential.T h e most i m p o r t a n t previous study of oxygen poisoning in adult fruit flies is by Williams and Beecher (1944). Clark et al. (1954Clark et al. ( , 1960Clark et al. ( , 1961 have studied particularly the sensitivity of insect p u p a e which, in special cases at least, are irreversibly d a m a g e d by as little as 1 rain exposure to 30 psi of oxygen. Goldsmith and S c h n e i d e r m a n (1956) have also observed the poisoning of p u p a e of Morrnoniella by 1.6-10 a t m of oxygen. In some respects our findings m e r e l y confirm those of Williams and Beecher, b u t the life-span at a wide r a n g e of different oxygen tensions a n d the effects of intermittent exposures have not been previously investigated. It does not seem to have been d e m o nstrated previously, for a n y species, t h a t the oxygen tension in air at sea level is, in fact, optimal for survival. O u r studies of the rates of recovery f r o m oxygen poisoning and the effects of high pressures of inert gases at different pressures of oxygen will be presented in later papers. M E T H O D SThe flies used in these experiments came from one culture of Drosophila melanogaster, the Swedish R wild type, obtained by Mr. R. C. Baxter of the Department of Radiation Biology, in 1956, from Cold Spring Harbor Laboratory. The flies were raised in t693

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