Objective: Although the myocardial force–interval and relaxation–interval relations are considered to be mechanical expressions of myocardial Ca2+ handling, correlation of these phenomena with altered Ca2+ kinetics in the intact state is limited. Thus, I sought to determine the impact of selective impairment of physiologic sarcoplasmic reticulum Ca2+ release, achieved by the use of the drug ryanodine, on these relations in the intact animal. Methods: Twelve dogs instrumented with left ventricular manometers and piezoelectric dimension crystals were studied before and after ryanodine (4 μg/kg intravenously). End-systolic elastance was measured at paced heart rates of 120–180 bpm to determine the force-frequency response. Mechanical restitution and relaxation restitution were determined by measuring contractile (single beat elastance) and relaxation (peak negative dP/dt) responses for beats delivered at graded extrasystolic intervals, with normalized responses expressed as a function of extrasystolic interval. Results: Ryanodine accelerated mechanical restitution (time constant 60.3±3.9 versus 81.7±10.1 ms, ppppConclusions: These findings suggest that in the intact animal, Ca2+ handling by the sarcoplasmic reticulum is a primary determinant of mechanical restitution and early relaxation restitution, but not late relaxation restitution. Conversely, ryanodine induced augmentation of the force-frequency response indicates a central role for sarcolemmal Ca2+ influx in producing frequency potentiation.