A system to measure left ventricular heat production phi iv in conscious human subjects is described. Using phi iv and the rate of energy supply to the myocardium, the mechanical efficiency of the left ventricle is calculated. Heat carriage in left ventricular myocardial blood flow phi cs is calculated from coronary sinus flow and the transcoronary temperature difference. Diffusional heat loss phi diff is derived from the ratio of areas under temperature/time curves recorded in the aorta and coronary sinus after a bolus injection of cold saline into the pulmonary artery. The sum of phi cs, phi diff and phi chem (the energy of endothermic reactions with haemoglobin) gives phi iv. The estimated errors in the derived values of heat production and mechanical efficiency are less than 17 per cent. At basal heart rates 17 patients with good left ventricular function had a transcoronary temperature difference of 0.16 +/- 0.03 degrees C (mean +/- standard deviation), left ventricular flow blood of 145.3 +/- 81.5 ml min-1, and phi iv of 3.1 +/- 1.8 W. Left ventricular oxygen extraction was strongly correlated with phi iv (r = 0.925, p less than 0.001). Left ventricular mechanical efficiency was 0.33 +/- 0.11, at basal heart rates, which fell to 0.18 +/- 0.15 (p less than 0.001) on atrial pacing at 100 beats min-1.