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1. A late inward current following the initial sodium current is sometimes observed on frog single skeletal muscle fibres studied in voltage-clamp conditions by the double sucrose-gap method. 2. The late inward current is time and voltage dependent; it is sensitive to extracellular sodium concentration and inhibited by tetrodotoxin. Therefore, it is very probably carried by sodium ions. 3. It is assumed that the late current reflects a regenerative increase of sodium conductance in the tubular membrane. The main argument favouring this assumption is that the late current is never observed on detubulated fibres. 4. Inactivation studies show a marked difference between the evolution of the two inward currents when the membrane resting potential is increased by previous hyperpolarizations in a high range of values: the late current is decreased while the initial current remains constant. The decrease of the late current is suppressed by tetraethylammonium ions or in a potassium-free medium. On detubulated fibres, the sodium current is never decreased by conditioning hyperpolarizations. 5. It is postulated that a fast transient potassium current, located in the bubular membrane, should develop contemporaneously with the sodium current. 6. The decay of the sodium inactivation is anaylsed on semilogarithmic plot: on normal fibres, it runs in two phases while on detubulated fibres, one phase only is found.