The effects of Ancylometes sp. venom on muscle contraction and bioelectrical potentials were investigated in the rat phrenic nerve diaphragm muscle preparation. The venom (50 microg/ml) depolarized the diaphragm muscle fiber membranes. This effect was abolished by tetrodotoxin and by reduction of the sodium concentration of the Tyrode solution. The increase in the frequency of miniature end plate potentials induced by the venom was also suppressed by tetrodotoxin (3 microM). These results indicate that the venom may activate voltage-dependent sodium channels in cell membranes. All of the effects of Ancylometes sp. venom on this nerve muscle preparation (i.e. increase in twitch tension, spontaneous small phasic contractions and increase in the frequency of miniature end-plate potentials) may be explained in terms of its action on sodium channels.