Na+-activated potassium channels (KNa) have been identified in cardiomyocytes and neurons where they may provide protection against ischemia. We now report that KNa is encoded by the rSlo2 gene (also called Slack), the mammalian ortholog of slo-2 in C. elegans. rSlo2, heterologously expressed, shares many properties of native KNa including activation by intracellular Na+, high conductance, and prominent subconductance states. In addition to activation by Na+, we report that rSLO-2 channels are cooperatively activated by intracellular Cl-, similar to C. elegans SLO-2 channels. Since intracellular Na+ and Cl- both rise in oxygen-deprived cells, coactivation may more effectively trigger the activity of rSLO-2 channels in ischemia. In C. elegans, mutational and physiological analysis revealed that the SLO-2 current is a major component of the delayed rectifier. We demonstrate in C. elegans that slo-2 mutants are hypersensitive to hypoxia, suggesting a conserved role for the slo-2 gene subfamily.