This study evaluated whether the time course of left ventricular (LV) pressure decay is consistent from beat to beat in the normal heart under tightly controlled experimental conditions. We determined the variability of LV isovolumic relaxation and compared it with that of other hemodynamic parameters. Pressure decay was evaluated using a monoexponential time constant (T), a half-time (T1/2), and an average rate (Ravg) in nine chronically instrumented dogs. To eliminate physical factors that could lead to variability, the dogs were studied at paced heart rates after autonomic blockade and during apnea. At a heart rate of 160 beats/min the coefficient of variation (SD/mean, expressed as a percent) was higher for T (4.7%, P < 0.005), T1/2 (5.0%, P < 0.005), and Ravg (3.2%, P < 0.005) than for dP/dtmax (1.9%), as well as for end-diastolic volume (1.2%), end-systolic volume (1.2%), or end-systolic pressure (1.8%). Similar differences were present at 200 beats/min. Pressure decay was also assessed during major loading shifts induced by rapid caval occlusion. Surprisingly, comparison of first and last beats did not show significant differences for T or T1/2 but did for all standard hemodynamic parameters and for Ravg. While the best correlation with a relaxation parameter and hemodynamic parameters during changing loading conditions was for Ravg, the correlations were not consistent in every case. We conclude that LV pressure decay shows marked variability, unrelated to the algorithm used to assess it. Ravg, a model independent parameter, may be a useful way to quantify LV pressure fall.