Although the kinetics of cardiac systolic force restitution have been well described, the restitution kinetics of left ventricular relaxation have not been examined. To define relaxation restitution behavior, we studied seven dogs chronically instrumented with left ventricular high-fidelity micromanometers and piezoelectric dimension crystals. After a priming period at a basic cycle length of 375 msec, test extrastimuli were introduced after a range of extrasystolic intervals (ESIs). Relaxation behavior of control and extrasystolic beats was characterized by the time constant of isovolumic relaxation, τ. Relaxation restitution can be described by two concatenated monoexponential curves, an early phase described by a rapid time constant and a late phase described by a slower time constant (TC1, 36.21±7.90 msec; TC2, 75.94±10.65 msec; p<0.05). The first phase of relaxation restitution parallels systolic force restitution over the same range and displays faster recovery (TC(s), 58.93±10.01 msec, p<0.05). Postextrasystolic restitution of test pulses after beats at fixed ESIs depends on the initial ESI. Relaxation recovery of postextrasystolic beats proceeds faster with smaller initial ESIs (TC1for ESI of 300 msec, 13.27±4.05 msec; TC1for ESI of 450 msec, 72.85±21.72 msec; p<0.0001). The monoexponential pattern of restitution was seen with model-independent descriptors of relaxation as well as with τ.