It was demonstrated that acute hypoxia increased muscle sympathetic nerve activity (MSNA) by using a microneurographic method at rest, but its effects on dynamic leg exercise are unclear. The purpose of this study was to clarify changes in MSNA during dynamic leg exercise in hypoxia. To estimate peak oxygen uptake (Vo(2 peak)), two maximal exercise tests were conducted using a cycle ergometer in a semirecumbent position in normoxia [inspired oxygen fraction (Fi(O(2)) = 0.209] and hypoxia (Fi(O(2)) = 0.127). The subjects performed four submaximal exercise tests; two were MSNA trials in normoxia and hypoxia, and two were hematological trials under each condition. In the submaximal exercise test, the subjects completed two 15-min exercises at 40% and 60% of their individual Vo(2 peak) in normoxia and hypoxia. During the MSNA trials, MSNA was recorded via microneurography of the right median nerve at the elbow. During the hematological trials, the subjects performed the same exercise protocol as during the MSNA trials, but venous blood samples were obtained from the antecubital vein to assess plasma norepinephrine (NE) concentrations. MSNA increased at 40% Vo(2 peak) exercise in hypoxia, but not in normoxia. Plasma NE concentrations did not increase at 40% Vo(2 peak) exercise in hypoxia. MSNA at 40% and 60% Vo(2 peak) exercise were higher in hypoxia than in normoxia. These results suggest that acute hypoxia augments muscle sympathetic neural activation during dynamic leg exercise at mild and moderate intensities. They also suggest that the MSNA response during dynamic exercise in hypoxia could be different from the change in plasma NE concentrations.