1. The monoamine transmitter 5-hydroxytryptamine (5-HT) and the peptide Phe-Met-Arg-Phe-amide (FMRFa), which appear to contribute to presynaptic facilitation and inhibition of the sensorimotor synapse in the abdominal ganglion of Aplysia, can modulate the frequency of spontaneous transmitter release at synapses formed between dissociated Aplysia sensory neurones and motoneurones in vitro. 2. 5-HT caused a decrease in the mean time interval between consecutive miniature EPSPs (mEPSPs), while FMRFa, applied either by itself or together with 5-HT, caused an increase in the mean time interval between consecutive mEPSPs. 3. Depolarization of the presynaptic neurone caused a decrease in the mean time interval between consecutive mEPSPs. This modulation required external Ca2+. 4. 5-HT and FMRFa were able to modulate spontaneous release when applied in saline solutions lacking Ca2+ and containing Ca2(+)-chelating agents, suggesting that the modulation of spontaneous release by 5-HT and FMRFa did not require a Ca2+ influx. Similarly, spontaneous release could still be modulated by 5-HT and FMRFa in saline solutions containing 1 mM-Cd2+, which blocked both the voltage-gated Ca2+ channels and the evoked transmitter release. 5. To prevent a rise in intracellular Ca2+, we buffered the concentration of Ca2+ in the presynaptic terminals by injecting into the sensory neurone the Ca2+ chelator 1,2-bis(o-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid (BAPTA). The injection of BAPTA blocked evoked transmitter release, suggesting that it acted as an effective buffer of Ca2+ in the terminals. However, spontaneous release could still be observed and was still modulated by 5-HT and FMRFa. This suggests that the modulation of spontaneous release does not require an elevation of intracellular Ca2+. 6. We propose that 5-HT and FMRFa can modulate the rate of spontaneous release directly by mechanisms that do not require changes in the intracellular concentration of Ca2+. These mechanisms might contribute an additional component to the presynaptic inhibition and facilitation of evoked transmitter release.