Four groups of six rats each were tested on the acquisition of shuttle avoidance at one of four levels of oxygen concentration between 21% (normal air) and 9%. Initially performance was poorer at 9% though with repeated testing the differences between the groups diminished. Subsequent testing of all groups at 9% O2 gave evidence that the reduction in the effect of oxygen deprivation could be attributed in part to physiological adaptation i.e. acclimatization.