Neurotransmitter release is frequently regulated by peptides that modulate neuronal calcium channels. Whole-cell recordings show that the ion permeability and voltage dependence of these channels are controlled by a membrane-associated pathway involving GTP-binding proteins. Here we use perforated-patch recordings to show that, in addition to this pathway, the peptide somatostatin inhibits the calcium current in chick ciliary ganglion neurons by a second soluble pathway involving a cyclic GMP-dependent protein kinase (cGMP-PK). This somatostatin inhibition of Ca2+ current did not desensitize and was not characterized by the slowing of Ca(2+)-current activation (kinetic slowing) observed in whole-cell recordings. When cGMP-PK was inhibited, somatostatin inhibition of Ca2+ current resembled that observed with whole-cell recordings. cGMP agonists mimic the effect of somatostatin only in perforated patch recordings. An endogenous cGMP-PK therefore forms part of the mechanism by which somatostatin induces a sustained inhibition of neuronal calcium channels.