SUMMARY Since rapid eye movement (REM) sleep deprivation has been reported to affect the neuronal excitability in the brain, it was hypothesized that a change in the neuronal membrane-bound Na-K ATPase activity might be at least one of the factors inducing such a change. Therefore, in this study rats were deprived of REM sleep by using the platform technique and enzyme activity was estimated in the whole brain, in different regions of the brain and in microsomal preparations. Deprivation was carried out for varying periods and suitable control experiments were conducted to rule out the possibility of nonspecific effects. The observation supported our hypothesis and showed primarily that the deprivation increased the enzyme activity in the rat brain. It showed also that the pons and the medulla were the first sites to be affected by deprivation. The probable mechanism producing such a change is discussed.