The mechanisms responsible for the loss of cell potassium during renal ischemia are poorly understood. The present studies examined the hypothesis that potassium channels are activated as an early response to hypoxia and contribute to potassium loss independent from an inhibition of active K+ uptake. Potassium flux in suspensions of freshly isolated rat proximal tubules was measured using an ion-selective electrode. Exposure of the tubules to hypoxia for only 2.5 min resulted in a rise in the passive leak rate of K+ but no decrease in active K+ uptake. The passive leak of K+ was associated with a 40% decrease in cell ATP content. The passive K+ efflux was inhibited by 5 mM Ba2+ (95%) and by 15 mM tetraethylammonium (85%) suggesting that K+ channels were the primary route of K+ movement. The effects of K+ channel blockade on the development of hypoxic injury were also examined. Tetraethylammonium and glibenclamide, an inhibitor of ATP-sensitive K+ channels, reduced hypoxic injury as assessed by the release of lactate dehydrogenase or measurement of DNA damage. These results suggest that activation of K+ channels is an early response to hypoxia and contributes to hypoxic renal injury.