Mitochondrial dysfunction has been implicated as the cause of irreversible injury in the ischemic heart. To circumvent artifacts associated with organelle isolation, mitochondrial function was studied in intact isolated, Ca2+-tolerant rat ventricular myocytes. After 30 min of anaerobic incubation, myocyte viability decreased from 76 +/- 1 to 33 +/- 4%. Basal O2 consumption rates (nanoatoms . mg cell protein-1 . min-1) were 17.1 +/- 1.3 in aerobic cells and 51.0 +/- 9.8 in anoxic cells. Carbonylcyanide-p-trifluoromethoxyphenyl hydrazone (FCCP)-stimulated rates were 65.5 +/- 9.2 and 84.5 +/- 15.3 in aerobic and anoxic cells, respectively. Respiratory control ratio was lower in anoxic cells: 2.3 +/- 0.3 versus 4.2 +/- 0.4 in aerobic cells. These data suggest that early anoxic mitochondrial injury is due to increased permeability of the inner membrane. Addition of pyruvate, malate, and FCCP to cells made permeable by digitonin resulted in similar maximal O2 consumption rates: 276.5 +/- 31.8 in aerobic and 299.3 +/- 31.9 in anoxic cells, suggesting the electron transport chain is intact in anoxic cells. For purposes of investigating whether anoxic mitochondrial dysfunction is secondary to cellular or mitochondrial Ca2+ overload, total cell Ca2+, cytosolic free Ca2+ levels (measured by null-point titration), and mitochondrial Ca2+ contents (measured as FCCP-releasable Ca2+) were measured. There were no differences in these three parameters between aerobic and anoxic cells, suggesting that mitochondrial dysfunction and irreversible hypercontraction of isolated cardiac myocytes exposed to 30 min of anoxia are not related to Ca2+ overload.