Aconiti tuber butanol fraction shows positive inotropic effect on the isolated atrium of rabbit heart. To investigate the mechanism, the effect on microsomal ATPase activity of rabbit heart is observed. The microsomal fraction which contains the Na+- and K+-activated ATPase in the presence of Mg++ is isolated from the left ventricle of rabbit heart. The microsomal ATPase activity is maximally stimulated at Na+ and K+ concentration of 100 mM and 10 mM respectively. Microsomal Na+-K+-activated ATPase is inhibited by ouabain and Aconiti tuber butanol fraction. Ouabain and Aconiti tuber butanol fraction depress Na+-stimulation on microsomal ATPase activity, and the inhibitory effects are not completely reversed at Na+ concentration of 300 mM. Also, K+-stimulation on microsomal ATPase activity is inhibited by ouabin and Aconiti tuber butanol fraction and the inhibitions are not compeletely reversed at K+ concentration of 30 mM. It is, therefore, suggested that the inhibitory effect of Aconiti tuber butanol fraction on the microsomal ATPase activity may contribute to leading to the positive inotropic effect.