The development of membrane ionic conductances of rat extensor digitorum longus (EDL) muscle fibers was studied in vitro using intracellular recordings. At 7-8 days after birth, the potassium conductance (GK) dominated the total membrane conductance while the chloride conductance (GCl) was very low. A rapid increase of GCl towards adult values was observed after few days (12-14 day old rats), whereas GK did not decrease up to day 23. Denervation at 7-8 days after birth suppressed the maturation of the electrical parameters measured, and 15 days after the nerve crush, GCl was just detectable. These results suggest that the maturation of the electrical properties, and in particular that of the resting chloride conductance in mammalian striated muscle fibers, occurs during the first weeks of postnatal life and is dependent on innervation.