1. Voltage clamped short (approximately 1.5 mm) muscle fibres of the frog can develop maximum tension of 4.3 kg/cm2. 2. The time course of contractile responses to prolonged depolarization is markedly dependent on the fibre membrane potential. With sufficiently long pulses the responses present a plateau and a spontaneous relaxation phase. 3. At room temperature (20--22 degrees C), and at membrane potentials of -10 mV the plateau duration is about 2 sec and the spontaneous relaxation rate is 0.50 sec-1. At membrane potentials of -35 mV the plateau duration is 4.6 sec and the spontaneous relaxation rate is 0.28 sec-1. 4. When the fibres are depolarized at room temperature with relatively short pulses (less than 2 sec), the contractile responses are cut short at the end of the pulse, and the fibres relax at a rate (11 sec-1) which is independent on the pulse amplitude and duration. 5. The relaxation rate after a short pulse can be affected by membrane potential only in the region near the contractile threshold where further release of contractile activator is expected to occur. 6. The time course of contractile responses to prolonged depolarization may be shortened by conditioning depolarization. 7. The system responsible for the release of contractile activator may be tentatively described by three states, resting, activated, and inactivated, in analogy with the model proposed by Chandler, Rakowski & Schneider (1976) to describe the possible configurations of the potential dependent charge movement in muscle.