The toxic fraction PhTx2 of the spider Phoneutria nigriventer was studied with a modified loose patch clamp technique on frog skeletal muscle. At saturating concentration (8 μg/ml) potassium currents were unaffected whereas there was a 7-fold increase in the time constant of sodium current inactivation (at −13 mV test potential). The time course of tail current deactivation was at least 3-fold slower than the control. The steady state (100 ms) inactivation and the conductance activation were shifted toward more negative potentials by 12.2 and 7.0 mV, respectively. The reversal of the sodium current was shifted 7.6 mV to more negative potential. We conclude that PhTx2 prolongs the inactivation and deactivation processes of sodium ion channels. These effects may account for the toxicity of PhTx2.