Abstract: The effect of tetrandrine (Tet)on Na⁺, K⁺ -ATPase and Mg²⁺-ATPase in rat myocardial microsomes was investigated in vitro. Under optimal condition, Tet did not influence Na⁺, K⁺-ATPase activity but concentration- dependently inhibited Mg²⁺-ATPase with an IC₅₀ of 179 μmol/L. At 10 or 100 μmol/L, Tet caused the concentration- inhibition curves for ouabain a parallel shift to the fight. Tet 100 μmol/L markedly increased the activity of Na⁺, K⁺ -ATPase under suboptimal K⁺ or excessive Ca²⁺ condition. However, it did not significantly increase the enzyme activity when the Na⁺ concentration was lower. A kinetic analysis showed that Tet increased the affinity of Na⁺, K⁺- ATP ase to ATP, but did not change the maximal velocity of the enzyme reaction.