1. The aim of the present investigation was to contrast the Ca2+ dependence of cardiac energy metabolism in two species with differential reliance on extracellular Ca2+ for excitation-contraction coupling. 2. We measured energy expenditure as the rate of oxygen consumption (Vo2) of isolated, Langendorff-perfused hearts of rats and guinea-pigs during KCl arrest. In parallel experiments, we indexed intracellular Ca2+ concentration ([Ca2+]i) of isolated right-ventricular trabeculae, using the Ca2+ fluorophore fura-2 and ratiometric spectrofluorometry. By varying extracellular Na+ concentration ([Na+]o), Vo2-[Na+]o and [Ca2+]i-[Na+]o relationships were constructed for each species. 3. Reduction of [Na+]o during K+ arrest caused pronounced species-dependent elevations of both Vo2 and [Ca2+]i. Despite the species dependence of both Vo2 and [Ca2+]i on [Na+]o, a single species-independent Vo2-[Ca2+]i relationship obtained. 4. We infer that elevation of the metabolic rate of the arrested heart above its basal value is determined primarily by [Ca2+]i and is not species dependent.