In retinal pigment epithelium (RPE) cells Ca2+ is involved in many important cellular tasks. Thus a disturbance of the Ca2+ - homeostasis in RPE cells may lead to serious functional impairments resulting in cell death. We showed that application of OH- radicals resulted in a biphasic [Ca2+ ]i increase. The initial, transient peak increased in a radical concentration- dependant manner. This Ca2+ increase could neither be inhibited by blocking the L-type Ca2+ -channel nor the reverse modus of the sodium/calcium exchanger NCX1. Thapsigargin decreased the first Ca2+ transient suggesting an involvement of intracellular Ca2+ -stores. A second, continuous [Ca2+ ]i increase was seen after terminating the OH- application in 33% of the experiments using 1.5x [OH-] and in 100% of the experiments using 2x [OH-] and was accompanied by a decreased cell survival. The application of Thapsigargin elevated the incidence of the 2nd Ca2+ increase in the 1.5x experiments to 75%. By using calpastatin, a specific blocker of calpain, the second Ca2+ increase could be abolished in 90% of the experiments using 2x [OH-]. Calpain is known to cleave NCX in a [Ca2+ ]i dependant manner preventing proper Na/Ca exchange. We conclude that extracellular OH- can lead to a [radical] dependant Ca2+ - activation and Ca2+ -overload. By rescuing NCX function the Ca2+ overload and subsequent cell death are abolished.