An identified pair of electrically coupled neurons, present in each segmental ganglion of the leech Hirudo medicinalis, was chosen as an experimental model of electrotonic synaptic transmission. We have found that the transfer of electrical signals between these coupled cells is reduced by perfusing the preparation with either one of the two endogenous neurotransmitters, serotonin and dopamine. The action of both serotonin and dopamine on the electrical coupling is reproduced by the iontophoretic injection of cyclic adenosine monophosphate (cAMP) into one of the coupled neurons. We propose that the two endogenous neurotransmitters modulate the transfer properties of this electrical synapse by increasing the intracellular levels of cAMP, possibly by activating the adenylate cyclase system.