Background: Vestibular reflexes, such as vestibule-sympathetic reflex, have been considered as an important physiologic mechanism to maintain peripheral vascular resistance in case of orthostatic stress. Vestibular function may play an important role not only in maintaining spatial orientation, but also in determining G-tolerance. The purpose of this study was to investigate the possible contribution of vestibular function to G-tolerance. Methods: Vestibular function was measured indirectly in 86 subjects by analyzing vestibulo-ocular reflex (VOR) using a rotating chair test. They were also exposed to +6Gz acceleration for 30 seconds with rapid onset run (1 G/sec). After G-exposure, they were classified into the Pass and the G-LOC group according to their success or failure to maintain consciousness during G-exposure. The VOR parameters of the G-LOC group were compared with those of the Pass group. Results: While VOR gain values of one subset of the G-LOC group (low gain G-LOC group, N=8) were significantly lower than those of the Pass group at rotation frequencies of 0.02, 0.04, 0.08, and 0.32 Hz, VOR gain values of the other subset of the G-LOC group (high gain G-LOC group) were significantly higher than those of the Pass group at all rotation frequencies. There was no difference of VOR phase value between the Pass and the G-LOC groups. Conclusion: This study suggests the vestibular function may be involved in determining G-tolerance. According to VOR gain values, we were able to recognize two distinct G-LOC subgroups compared with the Pass group. The difference between afferent or efferent vestibular flows and target organ response can be a possible explanation for different VOR parameters of G-LOC subgroups.