1. To investigate the mechanism whereby protein kinase C enhances secretory function in adrenal chromaffin cells, we examined the effects of 12-O-tetradecanoylphorbor-13-acetate (TPA) on Ca(2+)-induced catecholamine release from digitonin-permeabilized cells, resolving the release into a MgATP-dependent priming step and a MgATP-independent Ca(2+)-triggered step. Treatment with TPA selectively potentiated the priming activity of MgATP, with little increase in the MgATP-independent release. The potentiation by TPA of the MgATP-dependent priming was blocked by [Ser25]protein kinase C(19-31), a specific substrate of protein kinase C. Gö 6976, an inhibitor selective for protein kinase C alpha and beta isoforms, also blocked the potentiation by TPA. These results suggest that activation of protein kinase C, probably the alpha isoform, potentiates the MgATP-dependent priming step. 2. The antibody raised against GAP-43, a known substrate of protein kinase C, also potentiated the MgATP-dependent priming. The effect of TPA and that of the anti-GAP-43 antibody were not additive. Calmodulin, which binds to GAP-43 and inhibits its phosphorylation by protein kinase C, abolished the effect of TPA. Thus, the present results suggest that protein kinase C potentiates MgATP-dependent priming, at least in part, through phosphorylation of GAP-43.