Toxins from spider venom, originally purified for their ability to block synaptic transmission in Drosophila, are potent and specific blockers of Ca2+ currents measured in cultured embryonic Drosophila neurons using the whole-cell, patch-clamp technique. Differential actions of toxins from two species of spiders indicate that different types of Drosophila neuronal Ca2+ currents can be pharmacologically distinguished. Hololena toxin preferentially blocks a non-inactivating component of the current, whereas Plectreurys toxin blocks both inactivating and non-inactivating components. These results suggest that block of a non-inactivating Ca2+ current is sufficient to block neurotransmitter release at Drosophila neuromuscular junction.