The time-course of fall in isovolumic left ventricular pressure after maximum negative dP/dt was found to be exponential in both isovolumic and ejecting beats, characterized by a time constant, T. T was slightly shortened with higher heart rates, but not significantly affected by changes in ventricular volume or end-systolic fiber length. T was significantly shorter in ejecting beats compared to isovolumic beats. Ejection resulted in lower peak systolic pressure and smaller end-systolic volume, but T shortened more in beats with greater systolic fiber shortening. T was not significantly affected by calcium chloride or acetylstrophanthidin, but norepinephrine significantly shortened T. During recovery from ischemia, T increased significantly. T appears to be an index of the activity of the active cardiac relaxing system and is dependent on systolic fiber shortening. The study shows that T is independent of systolic stress and end-systolic fiber length, and minimally dependent on heart rate. The exponential time-course of pressure fall after max neg dP/dt allows characterization of isovolumic relaxation by T. T is a precise hemodynamic measure and can be used to predict the time-course of relaxation after mitral valve opening in the intact heart. The study also shows that T is influenced by systolic fiber shortening, and not by end-systolic fiber length or volume. The results suggest that T is a useful index of relaxation and may be used to assess the activity of the active cardiac-relaxing system.The time-course of fall in isovolumic left ventricular pressure after maximum negative dP/dt was found to be exponential in both isovolumic and ejecting beats, characterized by a time constant, T. T was slightly shortened with higher heart rates, but not significantly affected by changes in ventricular volume or end-systolic fiber length. T was significantly shorter in ejecting beats compared to isovolumic beats. Ejection resulted in lower peak systolic pressure and smaller end-systolic volume, but T shortened more in beats with greater systolic fiber shortening. T was not significantly affected by calcium chloride or acetylstrophanthidin, but norepinephrine significantly shortened T. During recovery from ischemia, T increased significantly. T appears to be an index of the activity of the active cardiac relaxing system and is dependent on systolic fiber shortening. The study shows that T is independent of systolic stress and end-systolic fiber length, and minimally dependent on heart rate. The exponential time-course of pressure fall after max neg dP/dt allows characterization of isovolumic relaxation by T. T is a precise hemodynamic measure and can be used to predict the time-course of relaxation after mitral valve opening in the intact heart. The study also shows that T is influenced by systolic fiber shortening, and not by end-systolic fiber length or volume. The results suggest that T is a useful index of relaxation and may be used to assess the activity of the active cardiac-relaxing system.