The analytical method of cardiac energetics, developed by H.Suga, has been extensively used to evaluate the relationship between mechanical contractility and energy consumption by the left ventricle both in experimental and in clinical studies. The end-systolic maximum elastance (Emax) is measured to indicate contractility of the heart, and the mechanical energy is obtained by the pressure-volume area of a cardiac contraction cycle (PVA), which well correlates with the oxygen consumption. Here, we aim at interpreting the cardiac energetics in terms of the cellular mechanisms. We constructed a heart model of spherical shape to apply the Laplace law in converting the wall tension to pressure. The wall tension was calculated by the contraction model of Negroni and Lascano (1996), which is driven by the Ca transient of the comprehensive cardiac myocyte model, Kyoto Model. The 'Laplace heart' was connected with preload and afterload, and the pressure-volume loop was constructed. Our results of simulation under an aerobic condition show that the Negroni & Lascano model well reconstructs the linear PVA–oxygen consumption curve. The PVA-independent oxygen consumption could be well simulated by the ATP hydrolysis via the Na/K pump and the Ca pump on the sarcoplasmic reticulum. [J Physiol Sci. 2006;56 Suppl:S130]