Objective  To explore the effects of acute or chronic hypoxia on the viscoelastic properties of polymorphonuclear leukocyte (PMN) of rats.   Methods  Hypobaric chamber was used to simulate high altitude condition. Fifteen adult Wistar rats were randomized into 3 groups: control

acute hypoxia (5 000 m for 3 d) and chronic hypoxia (5 000 m for 30 d). Micropipette aspiration technique was used to evaluate the biomechanical properties of PMNs of rats and the experimental results were fitted with a three-element standard linear solid model.   Results  The viscoelastic coefficients of

PMNs in acute hypoxic condition [K

1

=(86.66±26.84) Pa

K

2

=(399.89±22.03)Pa

  μ=(4.23±1.82) Pa·s

]

or those values in chronic hypoxic condition [K

1

=(179.61±26.31) Pa

K

2

=(472.12±30.36) Pa

μ=(9.21±2.65) Pa·s

]

were significantly higher than the corresponding value of PMNs in control(P0.05) [K

1

＝(39.52±16.06) Pa，K

2

=(148.85±9.06) Pa，μ=(2.28±1.11) Pa·s

]

. Compared with the control

the initial strain

maximum strain and the time constant in acute hypoxia was obviously decreased（P0.05)

but the time constant in chronic hypoxia had no difference.   Conclusion  These results suggest that acute or chronic hypoxia has strong impact on the biomechanical properties of PMN

involving the increased rigidity of cell membrane and the decreased deformation of cell.