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The murmur produced by acute graded constriction of the base of the pulmonary artery was studied in the anesthetized dog by means of intraluminal phonocardiograms recorded from the pulmonary artery. In addition, right ventricular and distal pulmonary artery pressures were recorded, and the cardiac output was determined for each degree of pulmonary artery stenosis utilized in the study. A small diamond-shaped midsystolic murmur is normally recorded from inside the pulmonary artery. This murmur increases in intensity and duration with constriction of the base of the artery. A constant relationship was not found between the degree of pulmonary stenosis or pulmonary flow and the intensity of the murmur. The intensity did correlate, in an essentially linear fashion, with the average velocity of flow through the constricted area. The timing of the peak intensity of the murmur did not change in any consistent fashion with increasing degrees of pulmonary artery stenosis. Cardiac output and the velocity of flow through the constricted area of the pulmonary artery tended to increase with moderate degrees of constriction. When the lumen of the pulmonary artery was reduced 40 to 45 per cent, cardiac output and pulmonary artery pressure fell below control levels. With further constriction, cardiac output and pulmonary flow decreased; however, the velocity of flow through the stenotic area remained relatively rapid.