Preganglionic nerve stimulation in the rat superior cervical ganglion (SCG) caused an increase in guanosine 3':5'-cyclic monophosphate (cyclic GMP) in a Ca2+-dependent manner. This increase was inhibited by oxyhaemoglobin, and blocked stereoselectively by an inhibitor of nitric oxide synthase, N(G)-nitro-L-arginine. Thus, nitric oxide or a similar substance appears to mediate the neuronal cyclic GMP response to synaptic activity in the rat SCG.