The effect of sulhydryl reagents on nonlinear membrane currents of frog skeletal muscle fibers has been studied using the triple Vaseline gap voltage-clamp technique. These compounds, which are known to interfere with depolarization contraction coupling, also appear to diminish intramembranous charge movement recorded with fibers polarized to -100 mV (charge 1). This effect, however, is accompanied by changes in the fiber membrane conductance and in most cases by the appearance of an inwardly directed current in the potential range between -60 and +20 mV. This current is reduced by both cadmium and nifedipine and does not occur in Ca-free solution, suggesting that it is carried by calcium ions flowing through regular calcium channels that are more easily activated in the presence of SH reagent. These changes in the membrane electrical active and passive properties decrease the quality and reliability of the P/n pulse subtracting procedure normally used for charge movement measurements. These effects can be substantially reduced by cadmium ions (0.1 mM), which has no effect on charge movement. When SH reagents are applied in the presence of cadmium, no effects are observed, indicating that this cation may protect the membrane from the reagent effects. The effects of -SH reagents can be observed by applying them in the absence of cadmium, followed by addition of the cation. Under these conditions the conductance changes are reversed and the effects of the SH reagents on charge movement can be measured with a higher degree of confidence. Maximum charge is reduced by 32% in the presence of 1.5 mM PCMB and by 31% in the presence of 2 mM PHMPS. These effects do not occur in the presence of DTT and in some cases they may be reversed by this agent. Charge 2, recorded in depolarized muscle fibers, is also reduced by these agents.