1. The individual effects of stroke volume and speed of ejection on the duration of the isovolumic relaxation phase were analysed in the canine left ventricle with a constant end-systolic residual volume.2. A new technique was employed to maintain the ventricular end-systolic residual volume at a desired constant value regardless of wide changes in stroke volume and speed of ejection in a given inotropic background.3. The duration of isovolumic relaxation, which was defined to be the time taken for ventricular pressure to fall from its end-systolic level to its 75, 50 and 25% levels, markedly decreased with increases in stroke volume. The reduction amounted to as much as 30-50% when stroke volume was increased from zero (isovolumic) to 20-25 ml.4. The degree of shortening of the duration of isovolumic relaxation was largely independent of changes in speed of ejection which ranged from about 100-800 ml./sec at a constant stroke volume of 15 ml.5. It was therefore concluded that stroke volume itself could be a major determinant of the duration of isovolumic relaxation.6. It was speculated that the mechanism of the observed phenomenon might be a manifestation of the uncoupling effect of muscle shortening on contractile state.