Simultaneous M-mode echocardiograms and external phonocardiograms were recorded in 15 healthy subjects to evaluate the genesis of the second heart sound. The onset of the second sound was found synchronous with the coaptation of the aortic valve cusps and a sharp vibration on the aortic wall. The closed valve was oscillating for 30 to 45 ms after the coaptation of the cusps. Magnified echocardiographic studies of the interventricular septum revealed a consistent, momentary quiver across the septal myocardium a mean of 4 ms after the onset of the second sound. In most subjects, a transient myocardial vibration was observed in temporal association with the first heart sound as well. The present observations suggest that the aortic valve closure initiates the production of the second heart sound, the main audible component resulting, however, from vibrations in the cardiac structures after the valve closure.