OBJECTIVES: High exercise intensity may be associated with reduced adherence to exercise programmes, possibly because it is perceived as aversive. However, several authors have suggested that an intensity as high as 60% or 70% of maximal aerobic capacity (VO(2max)) is necessary for exercise to elicit positive affective changes. To elucidate this discrepancy, the affective responses to increasing levels of exercise intensity were examined. DESIGN: In total, 30 volunteers rated their affect every minute as they ran on a treadmill while the speed and grade were progressively increased. METHOD: The methodology was unique in three respects: (1) affect was assessed in terms of the dimensions of the circumplex model instead of distinct affective states, (2) affect was assessed repeatedly before, during, and after exercise, not only before and after, and (3) exercise intensity was standardized across participants in terms of metabolically comparable phases (beginning, ventilatory threshold, VO(2max)) instead of percentages of maximal capacity. RESULTS: Pre-to-post-exercise comparisons indicated affective benefits in the form of increased energetic arousal and decreased tense arousal. During exercise, however, affective valence deteriorated beyond the ventilatory threshold and until VO(2max), a trend that reversed itself instantaneously during cool-down. CONCLUSIONS: Exercise intensity that requires a transition to anaerobic metabolism can have a transient but substantial negative impact on affect and this may, in turn, reduce adherence to exercise programmes.