The aim of this study was to characterize and describe the cortisol responses in athletes over a 24-h period on different days involving multiple exercise sessions of varied intensity. Seventeen endurance athletes volunteered to undergo three experimental treatment conditions: (a) a control day involving no exercise; (b) an exercise day with two sessions of high-intensity exercise; and (c) an exercise day with two sessions of moderate-intensity exercise. Significant changes (P<0·01) were found owing to the influence of both types of exercise during the daytime (high-intensity exercise producing a greater cortisol response than moderate intensity). At night after the moderate-intensity exercise (at selected times), cortisol response was significantly less than during the control condition at corresponding times (P<0·05). After high-intensity exercise (at selected times), cortisol levels were significantly less (P<0·05) than in the control condition at corresponding times. Additionally, certain night-time responses after the high-intensity exercise were significantly less than the moderate-intensity exercise responses at similar time points. The results indicate that daytime multiple exercise sessions produce suppressed cortisol levels at night, and the magnitude of this effect is dependent upon the intensity at which the daytime exercise is performed. The physiological mechanism inducing this effect upon cortisol levels at night is unclear from the present data.