Synaptic boutons of homozygous neuromuscular junctions labelled with the dye FM1-43 fail to destain at 32oC after K+ depolarisation, in contrast to wild-type, and FM1-43 dye uptake cannot be evoked by K+ stimulation at 32oC. Destaining at 32oC is stimulated by black widow spider venom and calcimycin. Endocytotic uptake of FM1-43 at 32oC is stimulated by black widow spider venom.