The effect of left ventricular asynchrony induced by right ventricular pacing on relaxation indexes was studied at rest and during exercise in seven conscious dogs instrumented for chronic measurements of left ventricular pressure, coronary blood flow, and arterial pressure and with right atrial and ventricular pacing electrodes. Increasing heart rate with atrial pacing resulted in an increase in both left ventricular maximum and minimum rates of pressure development, LV dP/dtmax and LV dP/dtmin, respectively, as well as in a decrease in the relaxation constant T. In contrast, increasing heart rate with ventricular pacing resulted in a decrease in LV dP/dtmax, a small increase in LV dP/dtmin, and a significant decrease in T. During exercise with heart rate kept constant with atrial pacing, both LV dP/dtmax and LV dP/dtmin increased and T decreased to the same extent as during exercise in sinus rhythm. In contrast, exercising during right ventricular pacing resulted in a significant increase in T, expressing a slowing of relaxation. It is concluded that increasing heart rate alone in the presence of asynchrony of LV contraction induced by abnormal electrical activation results in a depressed contractile response, while the relaxation phase is not significantly affected. However, during sympathetic stimulation, a condition where synchronization should be improved, the relaxation phase is considerably lengthened.