OBJECTIVES: The purpose of this study was to investigate the effect of acute hypoxia on left ventricular function during constant exercise. METHODS: Examined by 2D and Doppler echocardiography, 15 male Judo athletes (age: 20.6 ± 2.0 yr, height: 173.4 ± 2.0 cm, weight: 76.3 ± 9.8 kg, training history: 7.0 ± 3.0 yr) were invited to complete a 6-min supine cycling exercise at constant intensity (100 W) in normoxia and hypoxia simulated 2000m and 3000m respectively. The concentrations of arterial oxygen saturation during exercise were 92.4  1.6% at 2000m and 87.1 ± 2.1% at 3000m. RESULTS: At rest, heart rate (HR) and cardiac output (CO) increased significantly at 3000m, and the values of end systolic diameter (ESD) and fractional shortening (FS) at 3000m were lower than those of at 2000m and normoxia (p<.05). During exercise, HR, SV and CO elevated with the increase of altitude, and compared with normoxia, FS, ejection fraction (EF) (p<.05) and end diastolic diameter (EDD) were elevated whereas ESD reduced at 3000m (p<.05). In comparison to nomoxia after the constant cycling at 4- 6th minutes, hypoxic HR and CO increased significantly, and hypoxic FS and EF kept at higher level at 3000m(p<.05). CONCLUSION: The results suggest that acute hypoxia increases left ventricular function during a moderately constant supine cycling exercise.