The influence of the interval between beats on myocardial contractility was examined in conscious dogs previously instrumented with a left ventricular micromanometer and ultrasonic crystals measuring left ventricular major and minor diameters and parietal wall thickness during control state and after inflation of an aortic cuff. Ventricular contraction was measured during variations of diastolic intervals (DI) produced by respiratory arrhythmia or atrial pacing. During control before aortic stenosis, minor equator shortening (% delta L) increased slightly after long (L) DI compared with short (S) DI with a corresponding decrease of end-systolic stress. During aortic stenosis, LDI were associated with a significant increase of % delta L compared with SDI although end-systolic stress was larger. Thus end-systolic stress-diameter relations were not modified during control but they were significantly shifted to the left after LDI during aortic stenosis. Inotropic state is thus increased after LDI during acute pressure overload but not during control, suggesting that the restitution phenomenon is load sensitive.