Neurotransmitter release into the synapse is stimulated by calcium influx through ion channels that are closely associated with the transmitter release sites. This link may involve the membrane protein syntaxin, which is known to be associated with the release sites and to bind to the calcium channels. There is evidence that presynaptic calcium channels are downregulated by second messenger pathways involving G proteins. Here we use the patch-clamp technique to test whether calcium current is regulated by G proteins in a vertebrate presynaptic nerve terminal, and whether this regulation is affected by the linkage to syntaxin. The calcium current in the nerve terminal showed typical G-protein-mediated changes in amplitude and activation kinetics which were reversed by a preceding depolarization. These effects of the G protein were virtually eliminated if syntaxin was first cleaved with botulinum toxin C1. Our findings indicate that this sensitivity of the current to modulation by G proteins requires the association of the presynaptic calcium channel with elements of the transmitter release site, which may ensure that channels tethered at release sites are preferentially regulated by the G-protein second messenger pathway.