Objective To study the changes of baroreflex sensitivity (BRS) during head -up tilt test (HUT) in patients with vasovagal syncope (VS),and to examine the relationship between baroreflex sensitivity and neurohormonal factors. Furthermore, to investigate the effects of the changes of BRS on VS.Methods Forty- two patients with unexplained syncope (Among the 42 patients, there were 22 patients with positive HUT and 20 patients with negative HUT respectively) and 20 healthy volunteers (with negative HUT) underwent passive head-up tilt testing, Antecubital vein blood samples were taken before and after HUT, or at syncope. The fasting plasma endothelin ,serum nitric oxide (NO), serum NE were measured. The BRS was assessed on the basis of the linear regression slope the RR interval versus systolic arterial blood pressure during the increment in blood pressure after intravenous administration of phenylephrine. Results ( 1 )During the syncope, the BRS significantly reduced in HUT( + ) group than baseline. At the end of tilt, the level of plasma ET, serum NO in patients with positive HUT significantly increased compared with baseline or normal controls, and the plasma concentration of NE also had the trend of increase. ( 2 ) By multiple regression analysis, a significant negative correlation was found between baroreceptor sensitivity and the plasma ET, NO at the end of HUT in patients with positive HUT, but there was no relationship between BRS and NE. Conclusions During the syncope occure, the BRS in patients with VS decreased significantly compared with normal controls. The abnormal plasma ET, NO concentration might contribute to the mechanism of VS.