Key points• With each beat of the heart, the left and right ventricles must overcome substantially different arterial pressures in order to eject blood.• We have tested whether this difference in mechanical demand is reflected in different metabolic energy expenditure.• Our experimental preparations were trabeculae isolated from both ventricles; our index of metabolic energy expenditure was their heat production in response to electrical stimulation.• We found that the cost of activating contraction (i.e. the production of heat in the absence of force generation) was higher in trabeculae from the left ventricle, thereby conferring a lower mechanical efficiency on that ventricle.• Correction for the activation heat reveals the thermodynamic efficiency of the actomyosin crossbridges; whereas there was no interventricular difference in this fundamental property of cardiac muscle, its dependence on force development is baffling.Abstract We compare the energetics of right ventricular and left ventricular trabeculae carneae isolated from rat hearts. Using our work-loop calorimeter, we subjected trabeculae to stress-length work (W ), designed to mimic the pressure-volume work of the heart. Simultaneous measurement of heat production (Q) allowed calculation of the accompanying change of enthalpy ( H = W + Q). From the mechanical measurements (i.e. stress and change of length), we calculated work, shortening velocity and power. In combination with heat measurements, we calculated activation heat (Q A ), crossbridge heat (Q xb ) and two measures of cardiac efficiency: 'mechanical efficiency' (ε mech = W / H) and 'crossbridge efficiency' (ε xb = W /( H -Q A )). With respect to their left ventricular counterparts, right venticular trabeculae have higher peak shortening velocity, and higher peak mechanical efficiency, but with no difference of stress development, twitch duration, work performance, shortening power or crossbridge efficiency. That is, the 35% greater maximum mechanical efficiency of right venticular than left ventricular trabeculae (13.6 vs. 10.2%) is offset by the greater metabolic cost of activation (Q A ) in the latter. When corrected for this difference, crossbridge efficiency does not differ between the ventricles.