1. By alternating current stimulation, isolated uterus muscle of oestrogen-treated rabbit shows two peaks of tension development in relation to stimulating voltage. 2. This phenomenon can not be considered as a result of denaturation caused by heat production or pH change induced by high voltage stimulation. 3. The peak on the part of low voltage (the first peak) disappears by increasing K ion concentration in perfusing Krebs solution and reappears by washing the muscle with normal Krebs solution. 4. The same phenomenon identical in every detail can be observed with the rabbit rectum, one of the plane muscles, and/or toad's heart. 5. As regards the mechanisms of these phenomena, particularly that of the peak on the part of high voltage, the predecessor's opinion which stated that strong electric stimulations can activate the contractile system directly was criticized and was discussed for further developments on this problem.