We have found that cardiac temperature proportionally changes O₂ cost of contractility, defined as O₂ consumption for myocardial total Ca²⁺ handling normalized to contractility in terms of the end-systolic pressure-volume ratio (maximal elastance, E_max), in the canine left ventricle (temperature sensitivity, Q₁₀ = 2). We have separately found that a decrease in the recirculation fraction (RF) of Ca²⁺ within myocardial cells underlies an increased O₂ cost of E_max in stunned hearts. We therefore hypothesized that a similar change in RF would underlie the Q₁₀ of O₂ cost of E_max. We tested this hypothesis by analyzing RF calculated from an exponential decay component of the transiently alternating postextrasystolic potentiation in the canine left ventricle. RF decreased from 0.7 to 0.5 as cardiac temperature increased from 33 to 38°C with Q₁₀ of 0.5, reciprocal to that of O₂ cost of E_max. We conclude that Q₁₀ of ATP-consuming reactions involved in Ca²⁺ handling and E_max response to it could reasonably account for the reciprocal Q₁₀ of RF and O₂ cost of E_max.