The vesicular neuroexocytosis process consists of two important steps: fusion of transmitter-loaded vesicles at release sites on the presynaptic nerve terminal membrane; followed by the release of transmitter molecules into the synaptic cleft. We previously reported that in nerve growth factor (NGF)-differentiated PC12 cells, arachidonic acid (AA) release is associated with acetylcholine (ACh) release, botulinum neurotoxin A (BoNT/A) inhibits both processes and AA itself or a phospholipase A 2 (PLA 2 ) activator can cause ACh release in BoNT/A-poisoned cells in which SNAP-25 has supposedly been hydrolyzed. In the present study, we examined the roles of two endogenous intraterminal components in neuroexocytosis: the membrane fusogenic agent AA; and the vesicle fusion protein SNAP-25. A PLA 2 activator, mastoparan, was used to induce the release of AA and ACh from NGF-differentiated PC12 cells. Release depended upon the mastoparan concentration, as well as Ca 2ุ influx via the neuronal-type voltage-sensitive Ca 2ุ channels. Release of ACh followed a rise in intracellular free Ca 2ุ concentration; the increased Ca 2ุ activated PLA 2 and, thereby, increased the AA level. Scanning and transmission electron microscopy confirmed that mastoparan-induced ACh and AA release were not due to simple diffusion through damaged plasma membranes. Treatment of PC12 cells with appropriate antisense oligonucleotides blocked SNAP-25 expression, as judged by Western blot protein analysis with a specific monoclonal antibody. Despite apparent elimination of SNAP-25, treatment of differentiated PC12 cells with mastoparan and high (80 mM) K ุ induced ACh exocytosis. The results support the conclusion that PLA 2 and AA have important roles in neuroexocytosis that are independent of SNAP-25. Both PLA 2 and AA have been shown to be involved in actin cytoskeletal organization related to vesicle fusion and exocytosis. This mechanism may be an alternative target of BoNT/A other than SNAP-25.