Abstract-The contributions of increases in circulating catecholamines, changes in central command, and muscle afferents on baroreflex control of the sinus node during exercise are unclear. We used a dobutamine infusion to induce hemodynamic changes comparable to those of moderate physical exercise in the absence of changes in central command and muscle afferents in 13 healthy subjects. Dobutamine (up to 9 g/kg body weight per minute) increased systolic blood pressure, shortened the RR interval, increased systolic blood pressure variability, but blunted RR interval variability (PϽ0.05 versus placebo). Consequently, dobutamine decreased the coherence between variations in systolic blood pressure and RR interval and decreased arterial baroreflex sensitivity from 12Ϯ2 to 3Ϯ1 ms/mm Hg (PϽ0.01). The largest increases in systolic blood pressure with dobutamine were paralleled by the greatest impairments in arterial baroreflex sensitivity (0.50ϽrϽ0.56, PϽ0.01). The chronotropic effects of dobutamine prevented a reflex bradycardia in response to the blood pressure increase. However, less predominant low-frequency oscillations in systolic blood pressure (PϽ0.0001) suggested preserved sympathetic withdrawal in response to the blood pressure increase induced by dobutamine. In conclusion, this study revealed that a shift in the operating point of the arterial baroreceptors and the chronotropic effects of adrenergic stimulation impair baroreflex control of the sinus node during dobutamine exercise stress testing. Baroreflex control of the sinus node is not reset when hemodynamic characteristics of exercise are reproduced in the absence of modifications in central command and muscles afferents. (Hypertension. 1999;33:987-991.)