The effects of hemicholinium-3 (HC-3) on acetylcholine (ACh) release were studied on central inhibitory or excitatory synapses of Aplysia californica. HC-3 was used at concentrations below 10(-5) M, which did not affect choline uptake by this preparation. Statistical analysis of the synaptic noise evoked by sustained depolarization of the presynaptic neuron allowed us to calculate the amplitude and mean duration of the miniature postsynaptic responses at an inhibitory synapse in the buccal ganglion. Taking into account the modifications of miniature and evoked responses, it was concluded that HC-3 potentiates ACh release. A similar presynaptic effect was observed at an excitatory synapse in the abdominal ganglion. This facilitation of ACh release was prevented by tubocurarine or hexamethonium, pointing to an agonistic action of HC-3 on nicotinic presynaptic receptors implicated in a positive feedback on ACh release. The possible blockage of muscarinic presynaptic receptors by HC-3 was also considered. Hemicholinium-15 was without effect on ACh release but was nevertheless able to prevent the presynaptic action of HC-3.