Erythrocyte Na–K ATPase and sodium pump site numbers were estimated in groups of depressed, manic and recovered patients. The activity of Na–K ATPase per pump site was lower in the depressed group than in the recovered group. In the manic group Na–K ATPase was lower than that of the recovered group, whereas there was no difference in the pump site numbers. In the more severe manics the activity of Na–K ATPase per pump site was significantly lower than that of the recovered patients. Therefore, the change occurring in the erythrocyte membrane cation carrier in manic-depressive psychosis is probably in the activity of individual Na–K ATPase molecules and not in the number of Na–K ATPase molecules per cell.