The charges of left ventricular function provoked by coupled electrical stimulation were studied in 17 patients with severe chronic aortic incompetence and 9 controls. These results were compared with those obtained after isolated extrasystole in 24 cases of aortic incompetence and 10 controls, and during paired electrical stimulation in 12 cases of aortic incompetence. Left ventricular function increased uniformly after post-extrasystolic potentiation whatever the mode of stimulation. The preload did not change significantly, the increased ventricular function being mainly due to increased contractility. Coupled electrical stimulation mobilised the greatest contractility reserve (ejection fraction : +32 % in controls and +37 % in aortic incompetence and averaged circumferentiel fibre shortening +55 % in controls and +47 % in aortic incompetence). The contractility reserve is inversely proportional to the initial ejection fraction except when its value is less than 35 %. The myocardial response then becomes variable so that left ventricles with probable irreversible hypocontractility may be distinguished from those whose ventricular funciton would improve after surgical correction.