Studies on the effects of vanadate for Na-K-ATPase activity were carried out with rabbit renal cortex. 1) Na-K-ATPase activity was inhibited with the concentrations of vanadate in incubation medium. The vanadate concentration at which activity was inhibited by 50%(ID₅₀) was 10^-6M and Hill coefficient was 1.00. 2) The fractional inhibition by constant concentration of vanadate decreased with increasing enzyme concentration. 3) Increasing K⁺ and Na⁺ concentrations in incubation medium diminished the ability to inhibit Na-K-ATPase by vanadate whereas increasing K⁺ and Mg²⁺ concentrations potentiated the inhibition of Na-K-ATPase by vanadate. 4) Vanadate didn t inhibit Na-K-ATPase at pH 6.6. Increasing pH potentiated the inhibition of Na-K-ATPase activity. 5) Vanadate inhibited Na-K-ATPase activity reversibly in all range of concentrations in dilution experiment. These results show that vanadate inhibits Na-K-ATPase activity with interacting at KE² state reversibly.