Combined heat stress, dehydration, and exercise is associated with enhanced oxidative stress in humans, but the separate and combined effects of heat stress and exercise on circulatory markers of oxidative stress without the influence of dehydration remain uncertain. The purpose of this study was to determine the effects of whole body heat stress alone and in combination with exercise on blood markers of oxidative stress in euhydrated humans. Eight males wore a water-perfused suit at rest and during 6 min of one-legged knee extensor exercise under control and heat stress conditions while maintaining euhydration. Following the control trial and a 15 min resting period, hot water was perfused through the suit in order to increase core, skin, and mean body temperatures by ~1, ~6, and ~2°C, respectively. Blood samples were taken to measure reduced glutathione (GSH), oxidized glutathione (GSSG), superoxide dismutase (SOD) and plasma isoprostanes. Heat stress alone did not alter GSH, SOD activity, or plasma isoprostanes, but increased GSSG leading to a reduction in the GSH/GSSG ratio. No changes in these variables were observed with exercise alone. Conversely, combined heat stress and exercise increased both GSH and GSSG, decreased SOD activity, but did not alter GSH/GSSG ratio or isoprostanes. In conclusion, these findings suggest that heat stress, independently of dehydration, induces non-radical oxidative stress at rest but not during moderate exercise because an increase in antioxidant defense compensates the heat stress-induced non-radical oxidative stress.