(i) The aim of this study was to examine the influence of acute hypoxia on the urinary lactate response to endurance exercise and to study whether urinary lactate shows promise as an indicator of the hypoxic stress during endurance exercise in normobaric hypoxia. (ii) Eight healthy male subjects (mean±SD age 25±4 yr; height 171.0±4.5cm; body weight 65.3±6.9kg) performed two incremental exercise tests to exhaustion on a cycle ergometer in normoxia and acute normobaric hypoxia (FiO₂ = 14.4%). Thereafter, subjects undertook four bouts of constant-load exercise for 40 minutes at 40% and 60% maximal oxygen uptake (VO_2max) in random order on different days in the respective conditions. Urinary lactate were taken before, immediately after, and 30 minutes after each exercise. Blood lactate concentration and arterial oxygen saturation (SpO₂) were measured during exercise. (iii) Exercise at 40% VO_2max, the urinary lactate response to the endurance exercise in normoxia was significantly higher than hypoxia, while blood lactate concentration was no significant difference between normoxia and hypoxia. In contrast, no significant change was observed in the urinary lactate between two conditions at 60%VO_2max. SpO₂ was significantly lower in hypoxia than noromoxia at each exercise. Curve relation between urinary lactate and SpO₂ was seen at 60% VO_2max. (iv) These results suggest that normobaric hypoxia affect the relation between blood lactate concentration and urinary lactate response to the low-intensity endurance exercise. In addition, urinary lactate might be utility index to assess the degree of hypoxic stress during middle- intensity endurance exercise in normobaric hypoxia.