In rats treated with high-dose corticosteroids, skeletal muscle that is denervated in vivo (steroid-denervated [S-D]) develops electrical inexcitability similar to that seen in patients with acute quadriplegic myopathy. In studies of affected muscles in vitro, the majority of S-D fibers failed to generate action potentials in response to intracellular stimulation although the average resting potential of these fibers was no different from that of control denervated muscle. The downregulation of membrane chloride conductance (GC1) seen in normal muscle after denervation did not occur in S-D muscle. Although block of chloride channels in S-D muscle produced high specific membrane resistance, comparable to similarly treated control denervated muscle, and partially restored excitability in many fibers, action potential amplitude was still reduced in S-D fibers, suggesting a concomitant reduction in sodium current. 3H-saxitoxin binding measurements revealed a reduction in the density of the adult muscle sodium channel isoform in S-D muscle, suggesting that a decrease in the number of sodium channels present may play a role in the reduction of sodium current, although altered properties of channels may also contribute. The weakness seen in S-D muscle may involve the interaction of a number of factors that modify membrane excitability, including membrane depolarization, persistence of GC1, and reduced voltage-gated sodium currents.