The functional consequences of cholinergic self-inhibitory synaptic potentials (SISPs) upon firing patterns were examined in pairs of electrotonically coupled neurons of Aplysia buccal ganglia. In each neuron, the size of the peak SISP current decrements exponentially with increased number of previous conditioning action potentials (APs). To determine the effect of SISPs on the firing patterns of each cell, AP trains elicited by constant-current steps with the SISP intact were compared to those with the SISP blocked by curare. The SISP prolonged initial interspike intervals, providing an early supplement to accommodation, and produced a 75% increase in the sensitivity of firing frequency vs injected current plots for the first ISI. Firing rates were more regular in the presence of the SISP. However, the efficacy of the SISP, like the size of the underlying current, decrements with repetition. SISP effects were also studied in electrotonically coupled pairs of self-inhibitory neurons. Although the SISP altered the shape of the hyperpolarizing component of coupling potentials, DC coupling between the neurons was unaffected. Firing synchrony in coupled pairs stimulated with long DC pulses was assessed with cross-correlation histograms. In 60 mM Ca2+, the SISP sharpens the central peak of synchrony and deepens the flanking troughs, increasing the probability of synchronous firing within +/- 4 msec by 76%. The major determinants of synchrony were found to be common input, SISP-dependent regularity of firing, and the depolarizing phase of the coupling potential, rather than the SISP-enhanced hyperpolarizing phase.