Isolated pure cholinergic synaptosomes from Torpedo electric organ were incubated in vitro with beta-bungarotoxin for 15, 30 and 60 min and processed for electron microscopy. It was found that no morphological damage was seen after 15 min but by contrast, severe disruption of synaptosomes was present at 30 or 60 min after incubation with toxin. Synaptosomes were incubated also for 15 min in the presence of 125I-labelled beta-bungarotoxin and the binding was evaluated by electron microscopic autoradiography. The toxin was found to bind to the presynaptic membrane. The surface density of toxin binding sites was calculated to be around 3000/micron2. In a minor population of synaptosomes, the toxin was translocated into large vesicles suggesting that the toxin-receptor complexes underwent endocytosis in such vesicles. These results give further support to the view that inhibition of transmitter release by the toxin is produced by its action on plasma membrane.