Micromolar concentrations of the flavonoid kaempferol were found to efficiently block cerebellar granule cell (CGC) death through low K+-induced apoptosis, as demonstrated by prevention of the activation of caspase-3, internucleosomal DNA fragmentation, and chromatin condensation, without a significant rise in intracellular free Ca2+ concentration. Half of the maximum protection against CGC apoptosis was attained with 8 ± 2 μM kaempferol. Reactive oxygen species (ROS) were monitored with 2′,7′-dichlorodihydrofluorescein diacetate. Quantitative analysis of intracellularly and extracellularly oriented ROS production up to 3 h from the onset of low K+-induced CGC apoptosis was carried out with acquired digital fluorescence microscopy images of CGC in culture plates using a CCD camera, and also with fluorescence measurements of resuspended CGCs. In both cases, nearly 90% of ROS production by CGCs during the early stages (up to 3 h) after induction of low-K+ apoptosis occurs at the plasma membrane. Kaempferol, at concentrations that blocked CGC apoptosis, has been found to be a particularly potent blocker of extracellularly oriented ROS production by CGCs, and to inhibit the ascorbate-dependent NADH oxidase and superoxide anion production activities of the neuronal plasma membrane redox chain.