Intraarterial sound was measured just distal to the aortic and pulmonary valves of 10 subjects with no apparent valve disease. Six patients had no audible murmur; four had grade 1 to 2 innocent murmurs. At rest, during normal sinus rhythm, the intensity of intraarterial sound was greater above the aortic than above the pulmonary valve (0.41+/-0.14 versus 0.02+/-0.01 ergs/sec per cm2 [mean+/-standard error of the mean]) (P less than 0.02). In all patients with an audible murmur, the murmur was of greater amplitude within the aorta than within the pulmonary artery. The two patients with a grade 1 murmur had a murmur near the aortic valve and no murmur near the pulmonary valve. To examine the effects of increased flow, the six patients with inaudible murmurs were studied during the first beat immediately after a premature ventricular contraction. The intensity of intraarterial sound after premature contractions in these six patients was 1.41+/-0.38 ergs/sec per cm2 above the aortic valve and 0.10+/-0.04 above the pulmonary valve (P less than 0.01). The intensity of murmurs in the aorta during postextrasystolic beats was in the range that occurs with grade 1 to 2 murmurs, whereas murmurs within the pulmonary artery were in the range of inaudible murmurs. Comparable observations were made in dogs in which instantaneous flow was also measured. These observations suggest that innocent murmurs are produced at the aortic rather than the pulmonary valve, possibly because of the greater compliance of the pulmonary artery, which may have a damping effect upon turbulence.