The purpose of the present study was to investigate the effect of maximal interval training for 10 weeks on arterial oxygen saturation (SaO₂) and ventilatory response during heavy exercise. Seven subjects volunteered for participation in the study. All subjects performed an interval training 4 days per week. Training protocol per day consisted of five periods of exercise of 3-min duration on a cycle ergometer at a power output on 100% maximal oxygen uptake (VO₂max), interspersed with 2-min recovery cycle at 50%VO₂max. VO₂max and ventilatory data measured every week. SaO₂, end-tidal oxygen partial pressure (PETO₂), end-tidal carbon dioxide partial pressure (PETCO₂) and the ventilatory equivalent for oxygen (VE/VO₂) measured during 5-min heavy exercise at 90%VO₂max every other week. VO₂max significantly increased from 52.5±4.9 to 60.6±5.8 ml⋅ml^-1⋅kg^-1 during the training. SaO₂ reduced significantly from 95.4±1.1 to 93.3±1.8%, similarly PETO₂ and VE/VO₂ reduced during the training. A significant positive correlation was found among SaO₂, PETO₂ and VE/VO₂ during heavy exercise (SaO₂-PETO₂, r²=0.48, P<0.05 ; SaO₂-VE/VO₂, r²= 0.49, P<0.05; PETO₂ - VE/VO₂, r²=0.81, P<0.05) . These results suggest that-50% of the arterial O₂ desaturation during heavy exercise can be accounted for by low ventilatory response. Therefore the maximal intervsal training induced arterial O₂ desaturation during heavy exercise, which can be half explained by low ventilatory response.