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Presented in this paper is research designed in part to show that although moderate running by well-trained runners plays a significant role in reducing stress response to subsequently introduced stressors, running at the level of marathon competition alters psychological and physiological dispositions in a manner not conducive to reduced stress responses. In these demonstrations, the choice of our psychological and physiological dependent measures was guided by a larger theoretical framework concerning the relationship of exercise to temperament. We will discuss this larger view first because it provides a theoretical perspective that is useful in considering our specific hypotheses. We believe that regular aerobic exercise, with its requirements for sympathetic nervous system (SNS) activation and associated endocrine activity, leads to chronic reduction in the individual's experienced stress responses to psychological stressors. As long as two decades ago, Michael (1957) suggested that regular exercise would allow greater steroid reserves-reserves available to counter stress. Other similar views include that advanced by Edington and Edgerton (1976), who posit that extending the capacity of the adrenal medulla to generate the catecholamines through exercise may help to reduce the experience of stress. Moorehouse and Miller (1971) have suggested that exercise may "increase the size and lower the threshold of stimulation of the adrenal glands," resulting in greater reserves of antistress steroids and shorter response times to stressors.