Mitochondrial superoxide flashes reflect a quantal, bursting mode of reactive oxygen species (ROS) production that arises from stochastic, transient opening of the mitochondrial permeability transition pore (mPTP) in many types of cells and in living animals.However, the regulatory mechanisms and the exact nature of the flash-coupled mPTP remain poorly understood.Here we demonstrate a profound synergistic effect between mitochondrial Ca2+ uniport and elevated ROS production in triggering superoxide flashes in intact cells.