I schemia and reperfusion activate cardiac myocyte apoptosis, which may be an important feature in the progression of ischemic heart disease. The relative contributions of ischemia and reperfusion to apoptotic signal transduction have not been established. We report here that severe chronic hypoxia alone does not cause apoptosis of cardiac myocytes in culture. When rapidly contracting cardiac myocytes were exposed to chronic hypoxia, apoptosis occurred only when there was a decrease in extracellular pH ([pH]_o). Apoptosis did not occur when [pH]_o was neutralized. Addition of acidic medium from hypoxic cultures or exogenous lactic acid stimulated apoptosis in aerobic myocytes. Hypoxia-acidosis–mediated cell death was independent of p53: equivalent apoptosis occurred in cardiac myocytes isolated from wild-type and p53 knockout mice, and hypoxia caused no detectable change in p53 abundance or p53-dependent transcription. Reoxygenation of hypoxic cardiac myocytes induced apoptosis in 25–30% of the cells and was also independent of p53 by the same criteria. Finally, equivalent levels of apoptosis, as demonstrated by DNA fragmentation, were induced by ischemia-reperfusion, but not by ischemia alone, of Langendorff-perfused hearts from wild-type and p53 knockout mice. We conclude that acidosis, reoxygenation, and reperfusion, but not hypoxia (or ischemia) alone, are strong stimuli for programmed cell death that is substantially independent of p53.J. Clin. Invest. 104:239–252 (1999).