The neurotoxin Black Widow Spider Venom (BWSV) triggers massive release of neurotransmitter at synapses. Here we demonstrate that the action of BWSV on the frog neuromuscular junction can be visualized in vivo by the use of the fluorescent styryl dye FM1-43. This vital dye stains recycled synaptic vesicles upon nerve stimulation. Motor nerve terminals were stained with FM1-43 via electrical stimulation, washed and then exposed to BWSV or alpha-Latrotoxin. All terminals destained completely, independent of external calcium. Exposure of frog nerve terminals to BWSV in the presence of FM1-43 and calcium led to staining of terminals. The staining pattern appeared to be exactly the same as in control preparations, stimulated electrically via the nerve. When the same experiment was performed in the absence of calcium, only a minute quantity of dye was taken up into the nerve terminals, and the synapses looked swollen and puffed. Addition of external calcium to these preparations elicited an immediate shrinking of the nerve terminals, indicating endocytosis. These observations support electron-microscopic data that suggest an important role for extracellular calcium in endocytosis of BWSV poisoned nerve terminals.