The relationship between Ca2+ influx through voltage-activated Ca2+ channels, resting intracellular Ca2+ level (Ca-i) and Ca2+-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 Ca2+ (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.