1. When buccal neuron B2 of Aplysia californica is co-cultured with sensory neurons (SNs), slow peptidergic synapses are formed. When B2 is co-cultured with neurons B3 or B6, fast cholinergic synapses are formed. 2. Patch pipettes were used to voltage clamp pre- and postsynaptic neurons and to load the caged Ca2+ chelator o-nitrophenyl EGTA (NPE) and the Ca2+ indicator BTC into presynaptic neurons. The relationships between presynaptic [Ca2+]i and postsynaptic responses were compared between peptidergic and cholinergic synapses formed by cell B2. 3. Using variable intensity flashes, Ca2+ stoichiometries of peptide and acetylcholine (ACh) release were approximately 2 and 3, respectively. The difference did not reach statistical significance. 4. ACh quanta summate linearly postsynaptically. We also found a linear dose-response curve for peptide action, indicating a linear relationship between submaximal peptide concentration and response of the SN. 5. The minimum intracellular calcium concentrations ([Ca2+]i) for triggering peptidergic and cholinergic transmission were estimated to be about 5 and 10 microM, respectively. 6. By comparing normal postsynaptic responses to those evoked by photolysis of NPE, we estimate [Ca2+]i at the release trigger site elicited by a single action potential (AP) to be at least 10 microM for peptidergic synapses and probably higher for cholinergic synapses. 7. Cholinergic release is brief (half-width approximately 200 ms), even in response to a prolonged rise in [Ca2+]i, while some peptidergic release appears to persist for as long as [Ca2+]i remains elevated (for up to 10 s). This may reflect differences in sizes of reserve pools, or in replenishment rates of immediately releasable pools of vesicles. 8. Electron microscopy revealed that most synaptic contacts had at least one morphologically docked dense core vesicle that presumably contained peptide; these were often located within conventional active zones. 9. Both cholinergic and peptidergic vesicles are docked within active zones, but cholinergic vesicles may be located closer to Ca2+ channels than are peptidergic vesicles.