Objective. To investigate the changes of autonomic nervous system during acute exposure to an altitude of 5000 m by analysing heart rate variability (HRV). Method. 11 healthy male volunteers aged 18-30 were observed during inhalation of low oxygen gas mixture to simulate acute exposure to hypoxia. HRV was analyzed with both time domain and frequency domain methods. The eleven subjects were divided into two groups--Group A with good tolerance and Group B with poor tolerance. Result. During hypoxia heart rate increased markedly and RMSSD (the square root of the mean squared differences of successive RR intervals) decreased markedly; normalized low-frequency (LFn. u.) and LF/HF ratio increased significantly, while HF and normalized high-frequency (HFn. u.) reduced significantly. LFn. u. and LF/HF increased more apparently in group B than in group A during hypoxia of 5-10 min and RMSSD decreased more in group B during 10-15 min. Conclusion. The results suggested that cardiac sympathetic activity increased and cardiac vagal activity decreased during acute hypoxia. The analysis of HRV could predict the tolerance to hypoxia.