The left ventricular electromechanical interval (EMI) was measured in 50 normal subjects using both the apex upstroke (ACGu) and mitral component of the first heart sound (IM) as endpoints, and was compared with the kinetocardiographic endpoint (“Initial Ventricular Movement”). ACGu yielded the shortest EMI, for which our value is: EMI=q—ACGu=22 ± 9·8 msec. q—ACGu is the logical electromechanical interval, because ACGu is the earliest clinically detectable mechanical activity, preceding the LV pressure upstroke and occurring only slightly later than changes experimentally registered by intramyocardial sensors.