1. The electrical and the mechanical activity of isolated frog muscle fibres have been simultaneously recorded in a physiological solution which allows the development of a large tubular sodium current. 2. Under such experimental conditions, fibres develop long-lasting action potentials and strong mechanical responses. 3. In voltage-clamp experiments a slow inward current is revealed for depolarizations higher than +20 mV from the resting potential. This current increases until +40 to +50 mV and then decreases to reverse near +90 mV. The amplitude of the mechanical response increases with the potential to reach an optimum value between +40 and +50 mV and then decreases to stabilize when the depolarization is near +90 mV. 4. In the presence of picrotoxin the slow inward current is reversibly inhibited and the tension-depolarization curve has an S-shape as found in normal physiological conditions. 5. The dependence of a part of the contraction upon the slow inward current is reinforced by the fact that in a 50% sodium solution the amplitude of the current and that of the contraction are reduced in the same proportion. 6. Detubulated fibres failed to generate such a sodium inward current. 7. When sodium ions are replaced by lithium ions a slow inward lithium current develops but it does not induce a mechanical response. 8. Tetracaine reversibly inhibits the current-dependent component of the contraction without affecting the potential-dependent one. 9. It is concluded that the contraction recorded in the present experimental conditions is the sum of two components: one is potential-dependent and the other depends on a sodium-induced calcium release mechanism.