Contractile dysfunction plays a key role in injury sustained by ischemic myocardium at reperfusion, whereas interventions that impede hypercontracture enhance recovery. In permeabilized adult rat cardiomyocytes, the negative inotrope 2,3-butanedione monoxime (BDM; 10-50 mM) inhibited rigor at low MgATP concentration but stimulated net ATP hydrolysis. Hydrolysis was attenuated by H-7, kaempferol, chelerythrine, and genistein. Evidently BDM opposed phosphorylation of both serine/threonine and tyrosine kinase target proteins, either directly or by enhancing protein phosphatase activity, in a futile cycle of ATP hydrolysis independent of cross-bridge cycling. Although 20 mM BDM did not affect the onset of rigor contracture in permeabilized cells at low MgATP, in intact cells exposed to the metabolic inhibitors cyanide and 2-deoxyglucose rigor onset was accelerated, indicating that BDM increases ATP depletion in quiescent cardiomyocytes. Conversely, in cells exposed to the mitochondrial uncoupler carbonyl cyanide p-trifluoromethoxyphenylhydrazone, BDM delayed the onset of contracture and hence ATP depletion, consistent with an inhibition of adenine nucleotide movement across the mitochondrial inner membrane. Such effects will limit the value of BDM as a cardioprotective agent at physiological temperature.