Nigexine is a basic phospholipase A2 from the venom of the spitting cobra Naja nigricollis. In addition to its anticoagulant and cytolytic properties, nigexine also affects neuromuscular transmission in vitro. On chick biventer cervicis preparations, 1.5 microM nigexine caused a slowly developing block of responses to nerve stimulation, and a progressive loss of postjunctional sensitivity. Nigexine was at least 10 times less potent than notexin. On frog cutaneous pectoris preparations, nigexine caused a transient facilitation of evoked acetylcholine release, followed by a block. Spontaneous release was not abolished, and nigexine induced the release of abnormally large packets of transmitter. Nigexine also caused contracture of muscle fibres, accompanied by depolarization and degeneration. Nigexine appears to be able to cause prejunctional blockade and direct muscle damage to isolated skeletal muscle preparations.