The effect of omega-conotoxin GVIA, a specific blocker of N-type calcium channels, on the synaptic transmission at the mammalian neuromuscular junction is controversial. We have found that I-125-omega-conotoxin binds to rat tibialis muscle end-plate; that omega-conotoxin blocks the neuromuscular transmission both in vivo, in the sciatic nerve-tibialis anterior muscle, and in vitro in the isolated phrenic nerve-diaphragm preparation; and does not affect muscle nicotinic receptors. We conclude that in rat neuromuscular junctions N-type calcium channels are important for neurotransmitter secretion.