Isolated myocytes from ventricles are quiescent in the presence of 0.9 mM calcium. However, it is possible to induce beating by adding 0.5 mM BaCl₂ to the media or to induce a contracture by elevating the external concentration of potassium (72.5 mM K). During the viable stage of contracture, which is up to 1 h, the sarcomere spacing is 1.7 ± 0.1 μm and no leakage of intracellular components is observed. The metabolic properties of the cells in quiescent, beating and contracted states were compared. The O₂ consumption (natom per mg cell protein per min) increased from 10–11 in quiescent cells to 60–66 in beating cells and 90–99 in contractured cells. In contrast no significant difference was found in the metabolite levels in the three cellular states: (nmol per mg cell protein ± S.E.M.) ATP, 20.9 ± 1.5; CrP, 22.3 ± 2.2; ADP, 6.03 ± 0.67; Cr, 10.8 ± 1. It is proposed that the combined action of myosine ATPase, ATP synthase and cytosolic and mitochondrial creatine kinases serves to buffer the metabolite levels during periods of enhanced oxygen consumption.