Summary The concentrations of Mg2+, K+ and nitrogen in mitochondria isolated from dog myocardium after 2 and 3 hr of autolysis at 37° were significantly decreased from the control while the mitochondrial Ca2+ concentration showed a small apparent increase after 1 and 2 but not 3 hr of autolysis. Intramitochondrial dense bodies were found in the mitochondria isolated from damaged myocardium at all time intervals, but an increasing proportion of large mitochondrial fragments were isolated from the damaged tissue as the duration of autolysis increased, suggesting that the decreased nitrogen yield per unit quantity of heart was the result of increased mitochondrial fragility. These experiments do not finally establish whether the mitochondrial granules contain significant quantities of Ca2+, but do suggest that Mg2+ probably is not a significant constituent of the granules.