OBJECTIVE: To assess the role of autonomic nervous function during orthostatic tolerance tests (OTT) in pilots with history of G-induced loss of consciousness (G-LOC) or vasovagal syncope (VVS). METHOD: The relation between heart rate variability (HRV) and outcome of OTT result were compared among 3 groups of cases: 1) 30 patients with history of syncope and positive OTT as patient group (PG); 2) 20 cases with history of syncope but negative OTT result as negative group (NG); 3) 15 age-, sex-, flight plane model-, flight time-matched healthy pilots as healthy control (HC). OTT results and HRV, heart rate (HR), systolic blood pressure (SBP) and diastolic blood pressure (DBP) before and after OTT and their time and frequency domain were compared and analyzed among the three groups. RESULT: HR, SBP, and DBP were not significantly different (P>0.05) among the three groups before OTT. But during OTT, HR in PG was higher than those in NG and HC (P<0.01), SBP and DBP in PG were lower than those in NG and HC (P<0.01). 24 h continuous RR interval standard difference (SDNN), HRV triangle index (HRVI), and interval average value in PG were significantly lower than those in NG and HC; Power spectrum analysis of mean 1 h value for low frequency part (LF) and high frequency part (HF) were not significantly different among three groups, but LF/HF ratio in PG was higher than that in NG or HC (P<0.05). CONCLUSION: It is suggested that G-LOC and VVS may be due to increase in sympathetic tone and decrease in parasympathetic tone during orthostatic stimulation, as resulted from autonomic nervous dysfunction.