This study examined the effects of aerobic exercise on spontaneous electroencephalographic (EEG) activity. Participants (N = 34) were asked to (a) sit quietly for a 10-min adaptation period, (b) either exercise on a cycle ergometer (n = 18) or watch a videotape (n = 16) for 15 min, and (c) sit quietly for a 10-min recovery period. EEGs were collected during the last 2 min of the adaptation period, the last 2 min of each 5-min stage of the exercise/videotape period, and the last 2 min of the recovery period. EEG power densities were combined across the alpha and beta frequency bands. The results indicated that brain activation increased (i.e., alpha activity decreased and beta activity increased) during the exercise condition and returned to baseline following exercise. This did not occur in the nonexercise condition. Thus, the results were consistent with the opponent-process theory (Solomon, 1980) in that brain activation increased during exercise.