In order to assess the changes of HRV in relation to hemodynamic responses to LBNP, 40 healthy male volunteers were exposed to progressively increasing LBNP. Changes of SV, CO and BP were measured and HRV was analyzed. Thirty-four subjects (Group A, with good tolerance) were able to tolerate all stages of LBNP, and other 6 subjects (Group B, with poor tolerance) showed presyncopal signs and symptoms during the last 5min of LBNP exposure. In both groups, HRV, SV, CO decreased significantly during LBNP. Group B had lower HRV, SV, CO than group A pre-LBNP, during-LBNP and post-LBNP. The LF/HF ratio increased during LBNP in both groups, but increased more apparently in group B than group A at -6.7 kPa LBNP. The results showed that, when SV decreased to a certain degree, the sympathetic activity increased to the utmost. The onset of presyncopal signs could be attributed to the rapid withdrawal of sympathetic tone, and LBNP tolerance of a subject is closely related to both ejecting capability of the heart and the function of autonomic nervous system.