This study examined the impact of age on contractile function, Ca²⁺ homeostasis and cell viability in isolated myocytes exposed to simulated ischemia and reperfusion. Ventricular myocytes were isolated from anaesthetized young adult (3 mos) and aged (24 mos) male Fischer 344 rats. Cells were field-stimulated at 4 Hz (37°C), exposed to simulated ischemia and reperfused with Tyrode's solution. Cell shortening and intracellular Ca²⁺ were measured simultaneously with an edge detector and fura-2. Cell viability was assessed by Trypan blue exclusion. Ischemia (20-45 mins) depressed amplitudes of contraction equally in isolated myocytes from young adult and aged animals. The degree of post-ischemic contractile depression (stunning) was comparable in both groups. Ca²⁺ transient amplitudes were depressed in early reperfusion in young adult and aged cells and then recovered to pre-ischemic levels in both groups. Cell viability also declined equally in reperfusion in both groups. In short, some cellular responses to simulated ischemia and reperfusion were similar in both groups. Even so, aged myocytes exhibited a much greater and more prolonged accumulation of diastolic Ca²⁺ in ischemia and in early reperfusion when compared to myocytes from younger animals. In addition, the degree of mechanical alternans in ischemia increased significantly with age. The observation that there is an age-related increase in accumulation of diastolic Ca²⁺ in ischemia and early reperfusion may account for the increased sensitivity to ischemia and reperfusion injury in the aging heart. The occurrence of mechanical alternans in ischemia may contribute to contractile dysfunction in ischemia in the aging heart.