Electrophysiologic effects of physical exercise were investigated by comparing groups of individuals who engage in relatively low amounts of physical exercise (< 5 h/week) to subjects who engage in relatively high amounts of aerobic exercise (> 5 h/week). Event-related brain potentials (ERPs) were recorded using auditory and visual stimuli in separate oddball task conditions. P300 amplitude was affected by exercise frequency, such that increased amounts of exercise were associated with increased amplitude and somewhat more so for visual stimuli. No reliable exercise effects for P300 latency were observed, with little effect found for the other components. The findings suggest that a history of intensive physical exercise affects P300 amplitude. Theoretical mechanisms are discussed.