The modulatory role of protein kinase C on phospholipase A2, activation of which had been suggested to result in acetylcholine release from cholinergic neurons, was studied in longitudinal muscle preparations with the myenteric plexus of guinea pig ileum. The relationship of muscarinic autoinhibition to the modulation was also examined. Phorbol-12,13-dibutyrate (PDBu), an activator of protein kinase C, dose-dependently increased spontaneous and electrical field stimulation-induced acetylcholine releases from the preparation. The inhibitors of protein kinase C, staurosporine and calphostin C, inhibited the stimulatory effects of PDBu, but neither inhibitor affected spontaneous or electrical field stimulation-induced acetylcholine release in the absence of PDBu. On the other hand, atropine significantly increased electrical field stimulation-induced release by blocking a muscarinic autoinhibitory mechanism. Under the auto-inhibition blocked condition, U73122, an inhibitor of phospholipase C, and staurosporine significantly inhibited the effect of atropine on electrical field stimulation-induced release. An inhibitor of phospholipase A2, mepacrine, inhibited PDBu-induced acetylcholine release and also inhibited the effect of atropine on electrical field stimulation-induced release. An activator of phospholipase A2, melittin, and a product of the phospholipase, arachidonic acid, increased the spontaneous and electrical field stimulation-induced releases. These results suggest that the phospholipase C-protein kinase C system modulates acetylcholine release from cholinergic neurons by activating phospholipase A2 in the myenteric plexus of guinea pig ileum, and the activation of muscarinic autoreceptor may negatively modulate acetylcholine release at a point upstream of the system.