1. Recent developments which attempt to identify the mechanical determinants of myocardial oxygen consumption (mVO2) are considered, with emphasis being placed on the pressure-work and pressure-volume area (PVA) indices. 2. The difficulty of establishing a realistic in vivo basal mVO2 value is explained and the experimental reasons for the controversy over the magnitude of the activation metabolism are outlined. 3. The time varying elastance model of the heart is discussed including some current problems. The evidence for PVA as a satisfactory index of mVO2 under all physiological and pharmacological conditions is examined. Most pharmacological agents alter the intercept of the mVO2:PVA relationship but do not effect its slope: this result is interpreted to mean that the majority of current inotropic agents alter the energetic cost of calcium release/retrieval but not crossbridge efficiency. 4. An attempt is made to establish the likely cost of a cardiac contraction in man. The use of newer technology (i) to estimate mVO2 via positron emission tomography and (ii) to measure the work and potential energy output of the heart per beat with conductance catheters, is explained.