The neurotoxin, $\beta $-bungarotoxin ($\beta $-BuTX) purified from Bungarus multicinctus snake venom exerts various effects on frog and rat motor nerve terminals. Exposure to $\beta $-BuTX triggers, in the terminals, impulses which are rapidly distributed to the whole motor unit and elicit contractions of the muscle. $\beta $-BuTX raises the frequency of spontaneous release of acetylcholine quanta and inhibits the release evoked by nerve impulses, all of which leads to complete cessation of transmitter release. Subsequently, the motor axon disappears from the endplate and the Schwann cell establishes extensive synaptic contact with the muscle fibre. After only 30 min in $\beta $-BuTX, some endplates attain a stage of denervation similar to that seen several days after transecting the motor nerve. The neuromuscular effects of $\beta $-BuTX may result from its phospholipase activity and from an increase in the level of ionized calcium inside the terminals.