The relationship between Ca 2 + influx through voltage-activated Ca 2 + channels, resting intracellular Ca 2 + level (Ca i ) and Ca 2 + -dependent exocytosis was studied in bovine adrenal chromaffin cells by using patch-clamp, capacitance, and fluorescent measurements. It was established that depolarization-induced exocytosis passed over two steps, both of which linearly depend on Ca i . At Ca i lying below critical point (200-300 nM) the slope of the relationship was 4.43 and at Ca i exceeding the critical point the slope was equal to 31.63. The vesicular mechanism describing experimental two-step dependence of exocytosis on intracellular Ca 2 + (Ca i ) is proposed. According to the model at Ca i below critical point only small-sized vesicles fuse with plasma membrane, whereas at higher Ca i , larger vesicles started to fuse.