The author investigated the effect of Mg⁺⁺, Ca⁺⁺, Na⁺, K⁺ and creatine phosphate on the ATPase activity of microsomal fraction isolated from rabbit uterus and obtained the following results : 1) The uterine microsomal fraction contained the Na⁺- and K⁺- activated ATPase in the presence of Mg⁺⁺. The ATPase activity increased with protein content in the fraction. 2) The maximum ATPase activity was obtained at Na⁺ and K⁺ concentraction of 100 mM respectively. 3) In the absence of Mg⁺⁺, the ATPase was not activated by Na⁺ and K⁺, but inhibited. 4) Car stimulated the Na⁺- and K⁺- activated ATPase in the presence of Mg⁺⁺. However, in the absence of Mg⁺⁺, the ATPase was not activated by Ca⁺⁺. 5) The K⁺- activated ATPase activity was greater than the Na⁺-activated ATPase under all conditions. 6) The Na⁺- and K⁺ activated ATPase activity was increased by addition of creatine phosphokinase and creatine phosphate to the reaction mixture.