Serum corticosteroid (S.C.) responses were compared in two groups of middle-aged male subjects, one sedentary, one active (both N = 11, mean age 44 years), who performed graded bicycle ergometer exercise before (pre) and after (post) a 4 month physical fitness program. Pre-training exercise involved low and high work intensities which required similar percentage of predicted Vo2 max for the sedentary and active groups. Post-training exercise at identical work loads which resulted in lower work intensities due to training effects, was followed by increased work loads demanding higher work intensities in order to accommodate for the increased work capacity of the respective groups. The sedentary group (pre) and the active group (post) showed significant elevations in S.C. during high intensity exercise and recovery. The active group had significantly lower S.C. during low and high intensity exercise and recovery (pre), and during rest and low intensity exercise (post). The sedentary group had significantly lower S.C. at identical work loads which resulted in a reduction of relative work intensity from pre to post-traing tests. Results suggest both fitness status and training effects in the adrenocortical response to exercise stress, although psychological factors may be involved.