Studies of serum cortisol were performed on 11 male veteran athletes aged 36 to 67 yr. Venous blood was collected in a basal state, again immediately before competitive exercise, and after a 1500 m and a 5000 m race. The precompetition cortisol (28.9 ± 3.5 μg/100 ml) was twice that of the basal state (14.9 ± 4.0 μg/100 ml), p < 0.001, and there was an increase (p < 0.025) to 40.8 ± 3.3 μg/100 ml after the 1500 m race, and a further increase (p < 0.025) to 59.1 ± 4.9 μg/100 ml following the 5000 m race. In 2 subjects where the serum cortisol was markedly elevated prior to the exercise there was no change afterwards. The younger athletes were faster in both 1500 m (r–0.97) and the 5000 m races (r–0.94), but there was little correlation between age or speed and the pre- or postrace serum cortisol. The oldest subject aged 67 yr, studied before and after a marathon of 42.19 km, showed serum cortisols of 11.3 and 70.9 μg/100 ml, respectively. This study indicates that competitive athletics is a greater stimulus to serum cortisol elevation than comparable laboratory exercise and suggests that psychic and physical stresses are additive stimuli to cortisol secretion. The study also indicates that the response of the hypothalamic-pituitary-adrenal axis to physical exercise is unimpaired by age.