The fatigue of high-purity nickel in reversed bending has been studied as a function of oxygen pressure at temperatures of 20, 300, 425, and 550 C. The environmental effect was examined further as a function of strain at 300 and 550 C. S-shaped curves of fatigue life versus pressure obtained at each temperature were generally similar to one another in shape but did show some differentiating features. At all four temperatures there is a transition region in which fatigue lives drop fairly sharply with increases in oxygen pressure until a high pressure plateau is reached at what is termed the maximum transition pressure. This transition pressure is about the same at 300 C as at 20 C but progressively increases with further temperature increases. When these transition pressures are evaluated in terms of a theoretical model of the environmental effect, the agreement is found to be good at low temperatures. However, at high temperatures, the increase in transition pressure is much greater than would be predicted. This shift is attributed to oxidation along crack surfaces reducing the number of oxygen molecules which can reach the advancing crack tip. Supporting evidence is presented in the form of an activation energy calculation. While there was no apparent effect of strain on the maximum transition pressure at either 300 or 550 C, a higher strain decreased the magnitude of the environmental effect at the lower temperature. This behavior probably is related to strain hardening at the lower temperature which is not present at the higher temperature.