Neurons communicate by secreting a transmitter that excites or inhibits other neurons at synapses. The role of presynaptic membrane potential in triggering transmitter release is still controversial. In one view, presynaptic action potentials trigger the release by the entry of calcium ions into presynaptic terminals through voltage-dependent calcium channels. Calcium acts at high local concentrations at release sites near channel mouths to cause neurosecretion. An opposing view is that, in addition to elevating presynaptic calcium, presynaptic potential stimulates transmitter release by a distinct direct action. The relative importance of depolarization and calcium entry in neurosecretion cannot be determined because the two events are tightly linked. To delineate the roles of presynaptic potential and calcium entry in transmitter release, we have used nitr-5, a photolabile calcium chelator, and a voltage-clamp technique to control intracellular calcium and membrane potential independently at a synapse formed between cell bodies of cultured neurons of the fresh water snail Helisoma trivolvis. We found transmitter release occurred when presynaptic calcium levels were elevated to concentrations of a few micromolar, and that presynaptic voltage had no direct effect on neurosecretion.