AbstractInjecting cGMP intoAplysianeuron R14 induced an inward current similar to one elicited by application of FMRFamide to the outside of that cell. In contrast, injection of cAMP into R14 caused a long‐lasting outward current and conductance increase. Phosphodiesterase inhibitors increased the cGMP and FMRFamide‐induced inward currents in R14. The cGMP‐induced inward current is voltage dependent and is largely carried by Na+. It is also strongly and inversely dependent on both external [Ca2+] and [Cl−], although these ions are not significant current carriers. Changing external [K+] had no effect. Voltage and ion dependencies of the cGMP‐induced inward current are similar to those of an inward current induced by FMRFamide. Thus cGMP may be a second messenger to FMRFamide in producing a slow inward current in R14. cGMP does not appear to be a second messenger to FMRFamide in mostAplys