Energy metabolism of quiescent cardiac muscle was studied in the isolated rabbit heart preparation perfused at constant pressure by the Langendorff technique. Oxygen consumption (MVo2), coronary flow rate (CFR) and the steady state concentrations of high energy phosphate compounds were determined in hearts rendered asystolic using modified Krebs-Henseleit (KH) media containing 11 mM glucose as substrate. Basal MVo2 and CFR were significantly higher in hearts arrested by Ca2+ depletion (low Ca KH) compared to K+ excess (high K KH). Substitution of glucose in low Ca KH with a mixture containing glutamate, fumarate and pyruvate (low Ca KH + GFP) resulted in a 25% increase in the basal MVo2 but a 20% decline in CFR. Supplementing the low Ca perfusate with 30 g/l dextran (low Ca KH + dextran) depressed both the basal MVo2 (35%) and CFR (75%). Differences in the basal MVO2 under the different perfusion conditions were not accompanied by significant changes in the tissue levels of ATP, CrP or Cr. Compared to low Ca KH arrested hearts, those perfused with low Ca KH + GFP or low Ca KH + dextran did, however, show significantly lower tissue levels of ADP, AMP and Pi, but higher cytosolic ratios of [ATP]/[ADP][Pi] and [CrP]/[Cr][Pi]. As a consequence of the higher phosphorylation potential the free energy of ATP hydrolysis increased. There was no significant difference in any of these parameters between high K KH and low Ca KH perfused hearts. It is concluded that in the perfused, arrested heart none of the parameters that are used to describe the myocardial energetic state, e.g. free [ADP] or the cytosolic [ATP]/[ADP][Pi] ratio, uniquely correlates with the basal metabolic rate as estimated from MVO2 measurements.