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Restoration of blood flow to the ischemic myocardium prevents continuing cell necrosis, but reperfusion may cause irreversible damage to potentially salvable tissue, possibly through the generation of toxic reactive oxygen species. Intracellular calcium overload, secondary to membrane lipid peroxidation, has been proposed as a general pathogenic mechanism. However, using the photosensitisation of rose bengal to generate singlet oxygen and superoxide radicals, we demonstrate a direct effect of reactive oxygen species on the cardiac sarcoplasmic reticulum calcium-release channel. Exposure of heavy sarcoplasmic reticulum vesicles to reactive oxygen species in vitro resulted in the progressive loss of specific [3H]ryanodine binding and the degradation of high molecular weight proteins identified by polyacrylamide gel electrophoresis. The gating of single channels incorporated into artificial planar phospholipid bilayers was modified during the exposure to reactive oxygen species: an initial increase in open probability being followed by irreversible loss of channel function. Degradation by reactive oxygen species of specific proteins, such as the calcium-release channel, may contribute to in vivo reperfusion injury.