We have used membrane capacitance measurements to assay Ca²⁺-triggered exocytosis in single bovine adrenal chromaffin cells. Brief application of phorbol ester (PMA) enhances depolarization-evoked exocytosis severalfold while actually decreasing the Ca²⁺ current. Ca²⁺ metabolism is unchanged. Three different protocols were used to show that PMA increases the size of the readily releasable pool of secretory granules. PMA treatment leads to a large increase in amplitude, but little change in the time course of the exocytic burst that results from rapid elevation of [Ca²⁺]_i upon photolysis of DM-Nitrophen. Thus, PKC appears to affect a late step in secretion but not the Ca²⁺ sensitivity of the final step.