Abstract-The aim was to identify energy-consuming processes, other than contraction, downregulated during moderate hypoxia (Ϸ5 mol/L, 0.5% O 2 ) and severe hypoxia (Ͻ0.5 mol/L, Ͻ0.05% O 2 ) in isolated neonatal cardiomyocytes. The metabolic response of cardiomyocytes to moderate and severe hypoxia was assessed by measuring rates of energy consumption and energetic status of cells maintained under these conditions. We found that the rates of energy production were decreased during both forms of hypoxia. Decreased rates of energy production under moderate hypoxia were associated with reduced energy wastage through a downregulation of proton leak in the mitochondria. Cellular protein synthesis and RNA synthesis, major energy-consuming pathways, were downregulated only during severe hypoxia, when oxygen concentrations were low enough to induce energetic stress (quantitatively defined as being any situation in which phosphocreatine concentrations had fallen by Ն40% 3 has also been demonstrated experimentally in a variety of animal models. 4,5 These studies collectively suggest that hibernating cardiomyocytes maintain metabolic stability by reducing their rates of energy utilization to levels that match rates of energy production. It is not clear whether contraction is the only process downregulated during myocardial hibernation or whether any of the other energy-using processes are also shut down.The response of isolated contracting cardiomyocytes 6,7 to hypoxia (Ϸ8 to Ϸ29 mol/L, 0.8% to 3% O 2 ) is similar to the phenomenon observed in hibernating regions of myocardium, in that contractile activity is reduced and recovers with reoxygenation. We have previously shown that isolated noncontracting cardiomyocytes are also capable of reversibly downregulating metabolic activity during hypoxia (Ͻ5 mol/L, Ͻ0.5% O 2 ). 8 In the present study, we have assessed the effect of hypoxia on cellular energy consumption not associated with contraction by measuring the rates of protein synthesis, RNA synthesis, Na ϩ ,K ϩ -ATPase activity, and proton leak. Whereas hibernation was previously thought to be due only to reduced rates of contractile activity, 4 -7 in the present study, we show that rates of protein synthesis, RNA synthesis, and proton leak are also downregulated by noncontracting cardiomyocytes exposed to reduced oxygen availability.