Fifteen patients with symptomatic mild-to-moderate and severe chronic aortic regurgitation (AR) performed supine bicycle exercise while measurements of rest and exercise hemodynamics and left ventricular function were obtained. A continuous Doppler method was used to determine the change in distribution of total left ventricular stroke volume between forward stroke volume and regurgitant volume (RgV) with exercise. The pulmonary arterial wedge pressure (PAWP) was lower in the mild-to-moderate AR group than in the severe AR group at rest (8 + 1.2 vs 19 + 3.6 mm Hg, p = 0.01) and during exercise (15 + 3.9 vs 30 + 4.3 mm Hg, p = .02). In all patients there were increases in heart rate (78 ± 4 to 96 + 5 beats/min, p < .001), forward stroke volume (41 + 2 to 46 + 2 mI/m2), and the cardiac index (3.1 -+ 0.2 to 4.4 0.3 liters/min-M2, p < .001), despite a fall in total left ventricular stroke volume index from 84 5 to 76 + 5 ml/m2 (p = .03). The systemic vascular resistance (SVR) decreased with exercise from 1277 + 72 to 1031 64 dynes-sec/cm5 (p < .001), and the RgV and regurgitant fraction (RgF) both decreased with exercise from 43 + 5 ml/m2 to 30 ± 4 ml/m2 (p = .002) and 0.50 -+- 0.03 to 0.37 ± 0.03 (p < .001), respectively. Left ventricular ejection fraction increased on exercise from 0.51 + 0.03 to 0.55 + 0.03 (p = .02) for the group, but it either decreased or failed to increase by at least 0.05 in seven of 13 patients. The change in ejection fraction on exercise was directly related to the change in SVR (r = .80, p < .001). We conclude that: (1) in patients with mild-to-moderate AR, the PAWP is generally normal at rest and exercise, (2) in most of those with severe AR, the PAWP is elevated at rest and increases significantly with exercise, which is the likely mechanism for dyspnea on exertion in these patients, (3) the cardiac index in both groups is normal at rest and increases on exercise, (4) the increase in cardiac output results from both an increased heart rate and forward stroke volume, (5) the increase in forward stroke volume results from reductions of RgV and RgF, (6) the RgV and RgF are decreased due to a decreased SVR, and (7) the ejection fraction response to exercise is variable and correlates best with changes in SVR with exercise.