Phorbol myristate acetate (PMA), the active principle of croton oil, is a potent platelet aggregating agent. Previous cytochemical and electron microscopic studies demonstrated that PMA caused selective labilization of platelet storage granules resulting in their conversion to distended vacuoles. Communication was established between the enlarged, almost empty vacuoles and the surrounding plasma through channels of the open canalicular system. The present study has explored the possibility that the effect of PMA on platelet storage organelles stimulates a process of secretion. Analysis of the release reaction in platelets after exposure to PMA revealed that significant amounts of serotonin and adenine nucleotides were secreted without loss of lactic dehydrogenase. Release took place well before the onset of irreversible aggregation. Inhibitors of platelet secretion or ADP induced aggregation could modify or prevent the irreversible clumping stimulated by small concentrations of PMA. Although the secretion induced by PMA differed from the release caused by other agents, the findings of the present study indicate that PMA is capable of triggering the platelet release reaction.