The effect of ischemic injury on calcium and magnesium distribution in dog myocardial cells was investigated in tissue damaged by occlusion of the circumflex branch of the left coronary artery for 60 minutes or for 40 minutes followed by 20 minutes of reperfusion of the damaged tissue by arterial blood. No significant change in the concentration of these cations was noted in permanently ischemic, irreversibly injured myocardial cells, but tissue calcium was markedly increased in cells killed by an episode of transient ischemia. Tissue water and sodium also were increased and magnesium was decreased significantly in the transient ischemia model. Investigation of the localization of the increased Ca--by cellular fractionation and chemical analysis as well as by electron miscroscopy and microincineration showed that much of it was localized in dense bodies within the mitochondria. Within the intramitochondrial dense bodies, the calcium appeared to be a precipitate of an as yet undefined form of calcium phosphate.