Our purpose was to determine whether hearts from mice bioengineered to lack either the M isoform of creatine kinase (MCK^−/− mice) or both the M and mitochondrial isoforms (M/MtCK^−/− mice) have deficits in cardiac contractile function and energetics, which have previously been reported in skeletal muscle from these mice. The phenotype of hearts with deleted creatine kinase (CK) genes is of clinical interest, since heart failure is associated with decreased total CK activity and changes in the relative amounts of the CK isoforms in the heart. We measured isovolumic contractile performance in isolated perfused hearts from wild-type, MCK^−/−, and M/MtCK^−/− mice simultaneously with cardiac energetics (³¹P-nuclear magnetic resonance spectroscopy) at baseline, during increased cardiac work, and during recovery. Hearts from wild-type, MCK^−/−, and M/MtCK^−/− mice had comparable baseline function and responded to 10 minutes of increased heart rate and perfusate Ca²⁺ with similar increases in rate-pressure product (48±5%, 42±6%, and 51±6%, respectively). Despite a similar contractile response, M/MtCK^−/− hearts increased [ADP] by 95%, whereas wild-type and MCK^−/− hearts maintained [ADP] at baseline levels. The free energy released from ATP hydrolysis decreased by 3.6 kJ/mol in M/MtCK^−/− hearts during increased cardiac work but only slightly in wild-type (1.7 kJ/mol) and MCK^−/− (1.5 kJ/mol) hearts. In contrast to what has been reported in skeletal muscle, M/MtCK^−/− hearts were able to hydrolyze and resynthesize phosphocreatine. Taken together, our results demonstrate that when CK activity is lowered below a certain level, increases in cardiac work become more “energetically costly” in terms of high-energy phosphate use, accumulation of ADP, and decreases in free energy released from ATP hydrolysis, but not in terms of myocardial oxygen consumption.