This study investigates the hemodynamic determinants of the time-course of fall in isovolumic left ventricular pressure in isolated canine left ventricular preparations. The pressure fall was studied under isovolumic beats or during prolonged isovolumic diastole after ejection. The results show that the pressure fall from the time of maximum negative dP/dt is exponential during isovolumic relaxation for both isovolumic and ejecting beats, characterized by a time constant, \( T \). Higher heart rates slightly shortened \( T \) from 52.6 ± 4.5 ms at 110/min to 48.2 ± 6.0 ms at 160/min. Higher ventricular volumes under isovolumic conditions resulted in higher peak left ventricular pressure but did not significantly change \( T \). \( T \) shortened from 67.1 ± 5.0 ms in isovolumic beats to 45.8 ± 2.9 ms in ejecting beats. In ejecting beats, peak systolic pressure was lower, and end-systolic volume was smaller. Beats with larger end-diastolic volumes exhibited less shortening but similar end-systolic volumes and peak systolic pressure. \( T \) shortened more in beats with greater systolic shortening. Calcium chloride and acetylstrophanthidin did not significantly change \( T \), while norepinephrine, which accelerates active relaxation, significantly shortened \( T \). During recovery from ischemia, \( T \) increased significantly from 59.0 ± 9.6 ms to 76.8 ± 13.1 ms compared to preischemic control beats. The study concludes that the time-course of isovolumic pressure fall after maximum negative dP/dt is exponential, independent of systolic stress and end-systolic fiber length, and minimally dependent on heart rate. \( T \) may be an index of the activity of the active cardiac relaxing system and appears dependent on systolic fiber shortening.This study investigates the hemodynamic determinants of the time-course of fall in isovolumic left ventricular pressure in isolated canine left ventricular preparations. The pressure fall was studied under isovolumic beats or during prolonged isovolumic diastole after ejection. The results show that the pressure fall from the time of maximum negative dP/dt is exponential during isovolumic relaxation for both isovolumic and ejecting beats, characterized by a time constant, \( T \). Higher heart rates slightly shortened \( T \) from 52.6 ± 4.5 ms at 110/min to 48.2 ± 6.0 ms at 160/min. Higher ventricular volumes under isovolumic conditions resulted in higher peak left ventricular pressure but did not significantly change \( T \). \( T \) shortened from 67.1 ± 5.0 ms in isovolumic beats to 45.8 ± 2.9 ms in ejecting beats. In ejecting beats, peak systolic pressure was lower, and end-systolic volume was smaller. Beats with larger end-diastolic volumes exhibited less shortening but similar end-systolic volumes and peak systolic pressure. \( T \) shortened more in beats with greater systolic shortening. Calcium chloride and acetylstrophanthidin did not significantly change \( T \), while norepinephrine, which accelerates active relaxation, significantly shortened \( T \). During recovery from ischemia, \( T \) increased significantly from 59.0 ± 9.6 ms to 76.8 ± 13.1 ms compared to preischemic control beats. The study concludes that the time-course of isovolumic pressure fall after maximum negative dP/dt is exponential, independent of systolic stress and end-systolic fiber length, and minimally dependent on heart rate. \( T \) may be an index of the activity of the active cardiac relaxing system and appears dependent on systolic fiber shortening.