Potassium depolarization of rat brain synaptosomes (containing incorporated 1-acyl-2-[14C]arachidonyl-phosphatidylcholine) stimulated endogenous phospholipase A1 (EC 3.1.1.32) and A2 (EC 3.1.1.4), as determined by the formation of [14C]lysophosphatidylcholine, [14C]arachidonate, and [14C]prostaglandins, and also stimulated the secretion of [3H]catecholamines. The phospholipase A2 stimulation, dependent on calcium, was elicited in resting synaptosomes by A23187 and was demonstrated with incorporated 1-acyl-2-[14C]oleoyl-phosphatidylcholine but not with incorporated [14C]phosphatidylethanolamine or [14C]phosphatidylserine. Inhibitors of phospholipase A2 [rho-bromo-phenacylbromide (10 microM), trifluoperazine (3 microM), and quinacrine (3 microM) reduced the potassium-stimulated [3H]catecholamine release from synaptosomes to 78, 39, and 55%, respectively, of depolarized controls. The addition of lysophosphatidylcholine increased the release of [3H]norepinephrine to levels observed with potassium depolarization, whereas lysophosphatidylethanolamine, lysophosphatidylserine, and sodium dodecyl sulfate were much less effective. Potassium stimulation of synaptosomes increased the endogenous levels of free arachidonic acid and prostaglandins E2 and F2 alpha. Indomethacin and aspirin decreased the amounts of prostaglandins formed, allowed the accumulation of free arachidonic acid, and diminished the potassium-stimulated release of [3H]dopamine. rho-Bromophenacylbromide inhibited the formation of prostaglandin F2 alpha. Addition of prostaglandin E2 inhibited, whereas prostaglandin F2 alpha enhanced the release of [3H]norepinephrine. These results suggest that calcium influx induced by synaptosomal depolarization activates endogenous phospholipase A2, with subsequent formation of lysophosphatidylcholine and prostaglandins, both of which may modulate neurosecretion.