Using a three-electrode voltage clamp method, the effects of 5mM picrotoxin on the inward rectifier of frog skeletal muscle fibers in a high-K, Cl-free solution were examined. The instantaneous inward rectifier in the T membrane was virtually completely abolished with picrotoxin for the membrane potentials between about +80mV and －90mV, which covers full voltage range related to excitation-contraction coupling. The picrotoxin-resistant and almost time-invariant sarcolemmal inward rectifier resided for the membrane potentials more negative than about －90mV
