The hemodynamic determinants of the time constant of left ventricular (LV) isovolumic pressure (P) decline were studied in 32 anesthetized dogs. The time constant, tau (an index of LV relaxation), was determined from the best exponential fit of the equation P = Poe-t/r, to LVP measured at 5-ms intervals during isovolumic relaxation; Po = LVP at maximum negative dP/dt and t = time. At a constant heart rate of 120 beats/min, tau was determined during steady-state increases in preload (volume expansion), increases in afterload (methoxamine infusion), reductions in afterload (nitroprusside infusion), and in variably afterloaded beats at a constant preload (single-beat interventions). tau was directly related to LV systolic pressure and length during the alterations in LV loading conditions, but tau was not closely related to the extent of fiber shortening. During isoproterenol infusion, relaxation was more rapid (tau), and following the administration of propranolol, relaxation was prolonged (tau). While data from the variably afterloaded contractions indicate the presence of systolic load-dependent LV relaxation velocity, the steady-state studies do not exclude the possibility that altered contractility through reflex or other mechanisms contributes to the observed changes in tau.