Purpose: The impact of temperature on the coagulative response to exercise has not been well described. The purpose of this study was to assess the response of plasma thrombin-antithrombin (TAT) to exercise during exposure to both hot and cold temperatures, and to compare those responses to exercise under normal, temperate conditions. Methods: Fifteen healthy male subjects (25.3 + 4.3 years) volunteered to participate in this study. Subjects completed maximal cycle ergometer exercise tests in three different temperatures (20°C, 5° or 8° C, and 30°C) in an environmental chamber. All tests were conducted in random order and separated by at least seven days. Blood samples were obtained before and immediately after exercise and analyzed by Elisa to determine plasma concentrations of thrombin-antithrombin complex (TAT). Results: Subjects demonstrated significantly elevated plasma levels of TAT in all three temperatures immediately after exercise (normal =1.04 ± 0.44 ng/ml, cold =1.34 ± 0.79 ng/ml, hot =1.18 + 0.95 ng/ml) when compared to baseline measures (normal = 0.45 ± 0.26 ng/ml, cold = 0.88 + 0.57 ng/ml, hot = 0.64 + g/ml). Subjects also showed significant elevations in TAT concentrations both before and after exercise in the cold temperature when compared with the normal temperature. There was no significant difference between the hot and normal temperatures. Conclusion: An individual's coagulation potential is increased following maximal physical exertion and may be further increased by exposure to colder temperature. Key Words: coagulation, physical exertion, temperature, thrombosis. / School of Physical Education, Sport, and Exercise Science