The effects of Phoneutria nigriventer venom on muscle contraction and bioelectrical potentials were investigated in the rat phrenic nerve-diaphragm muscle preparation. The venom caused a non-uniform depolarization of the diaphragm muscle fiber membrane. This effect was abolished by tetrodotoxin or reduction of the sodium concentration in the bath fluid. The increase in the frequency of miniature end-plate potentials induced by the venom was also suppressed by tetrodotoxin. These results indicate that the venom activates the voltage-dependent sodium channel in muscle and nerve cell membranes. All the effects of the venom on the phrenic nerve-diaphragm muscle preparation (i.e. increase in twitch tension, delay in twitch relaxation, initial tonic contraction of short duration, spontaneous small phasic contractions, blockade of neuromuscular transmission, repetitive firing in nerve and muscle fiber membranes) can be explained on the basis of its action in the sodium channel. Nearly all of these effects are caused by discharges of repetitive action potentials in the nerve and/or muscle fiber membranes.