Background The time constant of left ventricular (LV) relaxation derived from a monoexponential model has been widely used as an index of LV relaxation rate or lusitropism, although this model has several well-recognized problems. In the present study, we proposed a logistic model and derived a “logistic” time constant ( T L ) as a better alternative to the conventional “exponential” time constant ( T E ). Methods and Results A total of 189 beats (147 isovolumic and 42 ejecting beats) were investigated in seven canine excised cross–circulated heart preparations. We found that the logistic model fitted much more precisely all the observed LV isovolumic relaxation pressure–time [P(t)] curves than the monoexponential model ( P <.05). The logistic model also fitted well both the time curve of the first derivative of the observed P(t) (dP/dt) and the dP/dt–P(t) phase–plane curve. Like T E , T L indicated that volume loading depressed LV lusitropism and that increasing heart rate and ejection fraction augmented it. T L was independent of the choice of cutoff point defining the end of isovolumic relaxation; T E was dependent on that choice. Conclusions We conclude that the logistic model better fits LV isovolumic relaxation P(t) than the monoexponential model in the present heart preparation. We therefore propose T L as a better alternative to T E for evaluating LV lusitropism.