We investigated how changes in ventricular contractility and arterial properties associated with exercise influence the energy transmission from the left ventricle to the arterial system. On six chronically instrumented dogs preconditioned to run on a treadmill, we imposed exercise loads of various degrees by altering the speed and slope of the treadmill (up to 7 km/hr and 201% slope). We evaluated ventricular contractility by end-systolic elastance (Ees) and arterial properties in terms of the effective arterial elastance (Ea). Eawas estimated by the ratio of mean aortic pressure to stroke volume. With exercise, Eessignificantly increased from 7.6±1.7 to 10.9±2.6 mm Hg/ml (p<0.005), and Eatended to increase from 4.9±1.4 to 6.7±1.8 mm Hg/ml (p=0.068), whereas the ratio of Eato Eesremained fairly constant (from 0.69±0.26 to 0.63±0.21, NS). The mechanical optimality index, defined as the ratio of stroke work to its theoretically derived maximal value, was 0.93±0.07 at rest and 0.92±0.08 at peak exercise. Similarly, the metabolic optimality index, defined as the ratio of cardiac oxygen consumption to stroke work conversion efficiency and its theoretical maximal value, was 0.98±0.02 at rest and 0.99±0.01 at peak exercise (NS). We conclude that external work of the left ventricle of these dogs was at a near maximal level for a given preload during exercise as well as at rest without compromising the conversion efficiency of metabolic energy to stroke work.