Postexercise cardiac morbidity is noted both in the exercise testing laboratory and in the field, but the physiology of this phenomenon has been unclear. Plasma catecholamine levels were studied in ten healthy men at each work load during exercise testing and during the recovery period after exercise. Both norepinephrine and epinephrine levels increased in response to exercise, although the response was much more noteworthy for norepinephrine. In the recovery period after exercise, both catecholamine levels continued to increase, with the norepinephrine level increasing tenfold over baseline. Such increases may have profound effects, particularly for subjects with preexisting coronary disease.