S cells form a chain of electrically coupled neurons that extends the length of the leech CNS and plays a critical role in sensitization during whole-body shortening. This process requires serotonin, which acts in part by altering the pattern of activity in the S-cell network. Serotonin-containing axons and varicosities were observed in Faivre's nerve where the S-to-S-cell electrical synapses are located. To determine whether serotonin modulates these synapses, S-cell action-potential (AP) propagation was studied in a two-ganglion chain containing one electrical synapse. Suction electrodes were placed on the cut ends of the connectives to stimulate one S cell while recording the other, coupled S cell's APs. A third electrode, placed en passant, recorded the APs near the electrical synapse before they propagated through it. Low concentrations of the gap junction inhibitor octanol increased AP latency across the two-ganglion chain, and this effect was localized to the region of axon containing the electrical synapse. At higher concentrations, APs failed to propagate across the synapse. Serotonin also increased AP latency across the electrical synapse, suggesting that serotonin reduced coupling between S cells. This effect was independent of the direction of propagation and increased with the number of electrical synapses in progressively longer chains. Furthermore, serotonin modulated instantaneous AP frequency when APs were initiated in separate S cells and in a computational model of S-cell activity after mechanosensory input. Thus serotonergic modulation of S-cell electrical synapses may contribute to changes in the pattern of activity in the S-cell network.