The determinants of the rates of ventricular relaxation and contraction were studied in 11 intact and five isolated canine heart preparations. Peak positive and negative dP/dt could be dissociated by afterload and inotropic stimulation, even though both were shown to be determined by the inotropic state of the muscle and instantaneous muscle length. An understanding of the interrelationships between inotropy, end-diastolic volume, end-systolic volume, and negative dP/dt helps to explain the dissociative effects of afterload and inotropic stimulation.