We used the patch-clamp technique to study the effect of cGMP on the 18-pS K channel in the basolateral membrane of the rat cortical collecting duct. Addition of 100 μM 8-bromoguanosine 3′,5′-cyclic monophosphate (8-Br-cGMP) increased the activity of the 18-pS K channel, defined byNP _o, by 95%. In contrast, applying 8-bromoadenosine 3′,5′-cyclic monophosphate (8-Br-cAMP) has no effect on channel activity. The effect of 8-Br-cGMP was observed only in cell-attached but not in inside-out patches. Application of 1 μM KT-5823, an inhibitor of the cGMP-dependent protein kinase (PKG), not only reduced the channel activity, but also completely abolished the stimulatory effect of 8-Br-cGMP, suggesting that the 18-pS K channel is not a cGMP-gated K channel. Addition of H-89, an agent that also blocks the PKG, mimicked the effect of KT-5823. To examine the possibility that the effect of 8-Br-cGMP is the result of inhibiting cGMP-dependent phosphodiesterase (PDE) and, accordingly, increasing cAMP or cGMP levels, we explored the effect on the 18-pS K channel of IBMX, an agent that inhibits the PDE. The addition of 100 μM IBMX had no significant effect on channel activity in cell-attached patches. Moreover, in the presence of IBMX, 8-Br-cGMP increased the channel activity to the same extent as that observed in the absence of IBMX, suggesting that the effect of cGMP is not mediated by inhibiting the cGMP-dependent PDE. That the effect of cGMP is mediated by stimulating PKG was further indicated by experiments in which application of exogenous PKG restored the channel activity when it decreased after the excision of the patches. In contrast, adding exogenous cAMP-dependent protein kinase catalytic subunit failed to reactivate the run-down channels. We conclude that cGMP stimulates the 18-pS channel, and the effect of cGMP is mediated by PKG.