Objective: To elucidate the role of the autonomic nervous system （ANS） in acute mountain sickness （AMS） during the initial phase at acute high-altitude exposure. Methods: Ninety-nine healthy sea-level residents rapidly ascended to Tibet plateau （3 675 m altitude） by airplane from Chengdu plain （560 m altitude）. ANS function was tested in plain and day 2–4 in Tibet by heart rate variability （HRV）

cold pressor test （CPT）. AMS was evaluated by clinic symptomatic scores. All subjects were divided into non-AMS group （57

scores≤4） and AMS group （42

scores>4）. Results: Compared with non-AMS group

AMS group had higher standard deviation of normal to normal intervals （SDNN）

root mean square of delta RR （rMSSD）

low-frequency （LF） power

and normalized low-frequency （LFnu） power in plain （P<0.05）. After arrival at 3 675 m altitude

AMS group had greater reduction in percentage of delta RR>50 ms（PNN50）

rMSSD （P<0.01） and SDNN

LF

total power （TP） （P<0.05）. Although no significant differences in the increase of SP and DP during CPT were found between 2 groups in plain

the SP increase during CPT of AMS group was less than non-AMS group （P<0.05） at 3 675 m altitude. AMS symptomatic scores was not only positively correlated with SDNN

rMSSD

LF/HF in plain （P<0.05）

but also negatively correlated with HFnu in plain （P<0.05）. Conclusion: During the initial high altitude exposure

ANS modulation is generally blunted

but the relatively predominant sympathetic control is enhanced

and this characteristic change of ANS function is positively correlated with the development of AMS.