Background and Objectives The arterial baroreflex has sympathoneural and cardiovagal efferent limbs. Whether quantitative measures represent interchangeable indicators of a single physiological construct has been unclear. Moreover, the extent to which results agree in the synucleinopathies pure autonomic failure (PAF), Parkinson disease with or without orthostatic hypotension (PD + OH, PD No OH), and multiple system atrophy (MSA) remains incompletely understood. In a retrospective observational study we comprehensively assessed physiological and neurochemical biomarkers of arterial baroreflex function in these disorders. Methods Data for 10 baroreflex-sympathoneural and 6 baroreflex-cardiovagal indices were compared among 38 patients with PAF, 63 with PD + OH, 63 with PD No OH, and 72 with MSA, along with 44 controls with complete datasets. Results Indices of baroreflex-sympathoneural and baroreflex-cardiovagal function were only weakly correlated. Within the two domains, correlations across subjects were much stronger. Among sympathoneural measures, pressure recovery time after the Valsalva maneuver, the logarithm of the total baroreflex area, and the fractional orthostatic increment in plasma norepinephrine best distinguished patients with autonomic synucleinopathies from controls. Among cardiovagal measures, the Baroslope derived from Phase II of the Valsalva maneuver and the logarithms of low- and high-frequency heart rate variability during supine rest were the most informative. No baroreflex measure separated PAF from PD + OH or MSA. Conclusions Indices derived from blood pressure and interbeat interval responses during the Valsalva maneuver capture much of the physiologically meaningful variation in arterial baroreflex function and provide efficient assessment of the sympathoneural and cardiovagal limbs in autonomic synucleinopathies.