The effects of chemically synthesized omega-conotoxin GVIA (a neurotoxic peptide from Conus geographus) on synaptic transmission at the bullfrog sympathetic ganglion, frog neuromuscular junction and electric organ of the ray, Narke japonica, were studied. The synthetic toxin irreversibly suppressed synaptic transmission at these synapses by arresting the release of transmission from the nerve terminals without showing postsynaptic effects. This action of the toxin was effectively antagonized by high concentrations of extracellular Ca2+. The synthetic toxin irreversibly blocked the Ca2+-dependent action potential of bullfrog sympathetic ganglion cells. These results suggest that omega-conotoxin GVIA blocks synaptic transmission by interfering with the Ca2+ influx through the voltage-sensitive Ca2+ channel of the nerve terminal. These results indicate that the chemically synthesized omega-conotoxin GVIA acts exactly like the natural omega-conotoxin GVIA. Thus, the synthetic toxin can be used in place of the natural toxin as a useful probe for the voltage-sensitive Ca2+ channel in the nervous system.