We previously established a new measuring method of the myocardial O2 consumption of mechanically unloaded rat left-ventricular slices. O 2 consumption of unstimulated myocardium corresponds to basal metabolism. We have found O2 consumption of stimulated myocardium to include basal metabolism and O 2 consumption for Ca2+ handling in the excitation-contraction coupling, but not for crossbridge cycling. Thus, O2 consumption for the excitation-contraction coupling is obtained by subtracting basal metabolism from O2 consumption of the stimulated myocardium. We have shown that O2 consumption for the excitation-contraction coupling corresponds to 40% of basal metabolism. The purpose of the present study was to analyse the component of myocardial O2 consumption for the excitation-contraction coupling by this method. Blockade of the sarcoplasmic reticulum Ca2+ pump by thapsigargin (0.1-1 micro mol/l), or by cycloplazonic acid (10 micro mol/l), significantly reduced O2 consumption for the excitation-contraction coupling by 40 or 70% of the respective controls. Neither thapsigargin nor cyclopliazonic acid reduced basal metabolism O2 consumption. The magnitude of free shortening of the unloaded myocardial slices, quantified by slice surface area reduction, was small (about 1.5%) because of the lack of external preload. Thapsigargin (1 micro mol/l) and cycloplazonic acid (10 micro mol/l) markedly attenuated the already reduced free shortening. 2,3-butanedione monoxime (5 mmol/l) also largely suppressed the free shortening, although this agent did not alter the O2 consumption of either unstimulated or stimulated myocardium. Some residual cross-bridge cycling may occur without detectable O2 consumption. Our present energetic results revealed that the O2 consumption of myocardial slices for the Ca2+ handling in the excitation-contraction coupling was mainly used for the sarcoplasmic reticulum Ca2+ pump.