1. beta-Bungarotoxin and other snake toxins with phospholipase activity augment acetylcholine release evoked from mouse motor nerve terminals before they produce blockade. This action of the toxins is independent of their phospholipase A2 activity, but the underlying mechanism for the facilitation of release is unclear. To determine whether the toxins affect ionic currents at motor nerve terminals, extracellular recordings were made from perineural sheaths of motor nerves innervating mouse triangularis sterni muscles. 2. Perineural waveforms had a characteristic shape, with two major negative deflections, the first being associated with nodal Na+ currents and the second with terminal K+ currents. Block of the K+ currents revealed a Ca2+-dependent component. 3. During the facilitatory phase of its action, beta-bungarotoxin (150 nM) reduced the second negative component of the perineural waveform by 30-50%. 4. The reduction could be a consequence of a decreased K+ ion contribution or of an increase in the current carried by Ca2+. As beta-bungarotoxin had similar effects in solutions which contained no added Ca2+, it is unlikely to be acting on the Ca2+ current. Also, it is unlikely to be blocking the Ca2+-activated K+ current, which is suppressed in zero Ca2+ conditions. 5. Other prejunctionally active snake toxins (taipoxin, notexin and crotoxin) had similar effects to those of beta-bungarotoxin, but a similar basic phospholipase of low toxicity from cobra venom had no effect. 6. Thus, beta-bungarotoxin and related toxins block a fraction of the K+ current in the motor nerve terminals of mouse preparations. Such an effect could explain the facilitation of acetylcholine release caused by these toxins before the onset of presynaptic blockade. 7. In frog cutaneous pectoris preparations, f-bungarotoxin reduced endplate potential amplitude but had little effect on perineural waveforms. Therefore, the consequences of toxin binding must be different in frog terminals.