This study is to investigate the effect of prolonged exercise with and without a thermal clamp on leukocyte cell, stress hormones, cytokine and muscle damage responses. Fifteen healthy male volunteers ( means ± SD: age 22 ± 3yr; mass 75.8 ± 3.2kg; maximal oxygen uptake 55 ± 7ml/ min/kg) randomly completed four chamber trials of 1 hour each, in different environment and separated by 7 days. Trials were: 1. exercise induced heating (EX-Heating: temperature/ humidity, 38˚C/ 50%); 2. exercise with a thermal clamp (EX-Cooling: temperature/ humidity, 18˚C/ 50%); 3. passive heating (PA-H: temperature/ humidity, 38˚C/ 50%); 4. passive cooling (PA-C: temperature/ humidity, 18˚C/50%). EX-H and EX-C were composed of 1h treadmill runs at 80% Individual anaerobic threshold (IAT). Blood samples were collected at pre-post, and 1h post environments exposure. Compared to EX-H, exercise-induced increases in core temperature, heart rate, cortisol, human growth hormone (hGH)), Interleukin-6 (IL-6), leukocyte counts and creatine kinase (CK) and Myoglobin (Mb) were significantly (p0.01) more pronounced than in EX-C. These results suggest that the additional impact of elevated ambient temperatures on stress responses to endurance exercise in trained subjects seems to affect primarily the hormonal systems and resulting changes in leukocyte number, creatine kinase, Myoglobin and interleukine-6.