In this study, evidence was established that the arterial contractile protein extracted by us must be arterial natural actomyosin and its characteristic features were clarified. Replacement of KCl by NaCl in the medium was inclined to enhance the superprecipitation and the ATP-ase activity of arterial natural actomyosin and then the de-crease of NaCl concentration in the perfusate resulted in reduction of the contractility of arterial strip by vasoactive agents. These results may explain clinical and epidemiological finding that a high intake of NaCl leads to hypertension.