Fifteen healthy male subjects classified as sedentary (7) or active (8), exercised to exhaustion on a bicycle ergometer at a work load requiring 70% of their maximal aerobic capacity. Biopsy samples of the vastus lateral is muscle were taken at rest, at the time of fatigue, and after a 10 min recovery. Training increased , , work time to fatigue, skeletal muscle glycogen levels, and phosphorylase a activity (P < 0.01). An exhaustive submaximal exercise reduced the glycogen levels of the trained group to values similar to the fatigue levels of the nontrained subjects: the higher the initial glycogen levels, the longer the subjects were able to exercise before the onset of fatigue. Skeletal muscle phosphorylase a activities decreased with exercise to fatigue in all groups whereas phosphorylase b activities were maintained. Training had no effect on resting phosphorylase b activity.