Phorbol myristate acetate (PMA) but not its inactive analogue phorbol didecanoate modulated the release of [3H] serotonin by rat basophilic leukemia (RBL) cells stimulated by antigen-IgE complexes. Concanavalin A or the calcium ionophore ionomycin, suggesting that protein kinase C (PKC) was involved in the exocytosis process. The PKC inhibitor sphingosine markedly inhibited release. When the PKC content of RBL cells was diminished by a prior 24 h-exposure (long-term PMA-treated cells) to 50 or 100 ng/ml PMA, the release induced by the three secretagogues was also strongly inhibited. Since cell activation by PMA in different cell systems is accompanied by PKC translocation from cytosol to membrane, we studied the location of PKC in resting cells and its translocation by a 5 min-exposure to 100 ng/ml PMA. PKC was cytosolic in long-term PMA-treated and control RBL cells and its translocation occurred regardless of the total PKC cell content, showing a possible correlation between the level of functional PKC (susceptible to be translocated) and exocytosis. Taken together, these data suggest that PKC is involved in the controlling of exocytosis by different secretagogues.