In chromaffin cells, Ca 2+ binding to synaptotagmin-1 and -7 triggers exocytosis by promoting fusion pore opening and fusion pore expansion. Synaptotagmins contain two C2 domains that both bind Ca 2+ and contribute to exocytosis; however, it remains unknown whether the C2 domains act similarly or differentially to promote opening and expansion of fusion pores. Here, we use patch amperometry measurements in WT and synaptotagmin-7-mutant chromaffin cells to analyze the role of Ca 2+ binding to the two synaptotagmin-7 C2 domains in exocytosis. We show that, surprisingly, Ca 2+ binding to the C2A domain suffices to trigger fusion pore opening but that the resulting fusion pores are unstable and collapse, causing a dramatic increase in kiss-and-run fusion events. Thus, synaptotagmin-7 controls fusion pore dynamics during exocytosis via a push-and-pull mechanism in which Ca 2+ binding to both C2 domains promotes fusion pore opening, but the C2B domain is selectively essential for continuous expansion of an otherwise unstable fusion pore. patch amperometry | synaptic transmission | capacitance measurements | neurotransmitter release | patch clamp