Recently, the psychological effects of exercise on cognitive function have been well documented. Most of these studies investigating the effect of exercise on event-related brain potentials (ERP), particularly on the P300 components, have found some evidence for the relationship between exercise-induced arousal and cognitive performance improvement. However, the often contradictory findings of experimental research have led several authors to identify four methodological factors to control in such studies: (i) the physical fitness of subjects, (ii) the intensity and duration of physical exercise, (iii) the nature of the psychological task, and (iv) the time at which the psychological task is administered to the subjects. The present study focused on the first and second factors. PURPOSE: The purpose of the present study was to investigate the interactive effects of exercise intensity and the physical activity level on human cognitive processing using P300. METHODS: Twenty six participants (23.96 ± 0.69 yrs.) were classified into two groups (active: 13, inactive: 13) by International Physical Activity Questionnaires (IPAQ). The physical activity level and maximal oxygen uptake between groups were significantly different. They performed a go/no-go reaction time task in a control condition (no exercise), and again after low -, medium -, and high – intensity pedaling exercises. The go/no-go reaction time task consisted of a warning stimulus (S1) followed 2 sec. later by an imperative stimulus (S2). A binaural 2000 Hz tone was used for S1. For S2, green and red LEDs at the fixation point appeared randomly, with both colors appearing with the same probability. Subjects were instructed to press a button with their thumb as fast as possible whenever the green LED was presented, but were told not to respond to the red LED. RESULTS: The P300 amplitude after medium – intensity pedaling exercise was significantly larger than after high – intensity pedaling exercise only in the inactive group. The P300 amplitude after medium – intensity pedaling exercise in the inactive group was larger, and after high – intensity pedaling exercise was smaller than in the active group. CONCLUSIONS: The changes inP300 amplitude by differential exercise intensity observed only in the inactive group indicated that the inactive group is more sensitive to exercise intensity than the active group. When the exercise intensity is high, facilitative effects of cognitive processing by pedaling exercise are cancelled only in the inactive group. In other words, the active group is better able to withstand the detrimental effects of physical stress than the inactive group.