&NA;We studied whether the oxygen cost of mechanical energy is time‐invariant in the excised, cross‐circulated canine heart. The total mechanical energy generated by ventricular contraction can be quantified by the total pressure‐volume area (PVA) according to the time‐varying elastance model. In this model, mechanical energy generated until a specified time (t) during systole can be quantified by the partial pressure‐volume area, PVA(t). PVA(t) was obtained by quickly releasing ventricular volume at a varied time during isovolumic contraction. The quick release aborted further development of mechanical energy. We found that PVA(t) at a constant end‐diastolic volume linearly correlated with myocardial oxygen consumption (Vo2). This indicates that the oxygen cost of mechanical energy is time‐invariant. However, we also found that the slope of the Vo2‐PVA(t) relation decreased with increasing quick‐release speed. This indicates a decrease in Vo2by the quick release despite the same PVA(t). The time‐invariant oxygen cost of mechanical energy is consistent with the time‐varying elastance model of the ventricle, but the decreased Vo2with increasing quick‐release speed despite the same PVA(t) is not. (Circulation Research1989;64:764‐778)