The temporal relationship between the closure of the aortic valve (AoV) and the onset of the aortic component of the second heart sound (A2) was defined by simultaneous recording of AoV echogram, phonocardiogram (PCG), and electrocardiogram in 25 subjects. Ten subjects had no heart disease (normal); 15 suffered from various cardiac conditions other than AoV disease (patients). The point of coaptation (C) of the AoV cusps to the onset of A2, the C-A2 interval, was measured to the nearest 5 msec. in 125 cycles. Fifty-eight cycles had a C-A2 of 10 msec., and 47 cycles had a C-A2 of 15 msec. The remainder were distributed at intervals below 10 or above 15 msec. The average C-A2 interval was 11.48 +/- 0.38 msec. (mean +/- 1 S.D.). A similar distribution pattern was observed when the total number of cycles was divided into "normal" and "patient" groups. In 3 subjects, simultaneous equisensitive (catheter-tip micromanometer) left ventricular and central aortic pressures, PCG, and AoV echograms were recorded. C-A2 ranged from 0 to 10 msec.; the interval between left ventricular and aortic pressure at the level of the incisura-hangout interval-ranged from 8 to 20 msec. Inhalation of amyl nitrite in one subject produced a significant fall in arterial pressure, accompanied by prolongation of the hangout interval from 10 to 20 msec. and of the C-A2 interval from 0 to 8 msec. Thus, the C-A2 interval is an integral part of the hangout time. Data suggest that A2 does not originate from the coaptation of the aortic valve cusps per se, but is related to events that occur at the time of or slightly after coaptation.