Excessive K(+) efflux promotes central neuronal apoptosis; however, the type of potassium channel that mediates K(+) efflux in response to different apoptosis-inducing stimuli is still unknown. It is hypothesized that the activation of large-conductance Ca(2+)-activated K(+) channels (BKCa) mediates hypoxia/reoxygenation (H/R)- and ischemia/reperfusion (I/R)-induced neuronal apoptosis. Rat hippocampal neuronal cultures underwent apoptosis after reoxygenation, as assessed by morphologic observation, terminal deoxynucleotidyl transferase dUTP nick end labeling staining, and caspase-3 activation. Single-channel recordings revealed upregulation of BKCa channel activity 6 h after reoxygenation, which might be caused by elevated cytosolic Ca(2+). The K(+) ionophore valinomycin and the BKCa channel opener NS1619 induced neuronal apoptosis. Transfection of the BKCa channel α subunit into Chinese hamster ovary (CHO-K1) cells, which do not express endogenous K(+) channels, or into neurons will induce cell apoptosis, indicating that the opening of the BKCa channel serves as a pivotal event in mediating cell apoptosis. The specific BKCa channel blockers charybdotoxin and iberiotoxin and the nonselective K(+) channel blocker tetraethylammonium at concentrations more specific to the BKCa channel were neuroprotective. The A-type potassium channel blocker 4-aminopyridine and apamin, a small-conductance Ca(2+)-activated K(+) channel blocker, were not protective. This result suggests the involvement of the BKCa channel in H/R-induced apoptosis. Similarly, specific BKCa channel blockers also showed neuroprotection in neurons subjected to oxygen-glucose deprivation/reoxygenation or animals subjected to forebrain ischemia-reperfusion. These results demonstrate that the over-activity of BKCa channels mediates hippocampal neuronal damage induced by H/R in vitro and I/R in vivo.