Background: A considerable amount of literature attests to the benefits associated with regular exercise on physiological well being (Berlin & Colditz, 1990: Am J Epidemiol 32, 62–628). However despite this many people do not partake in the minimum recommended amount of exercise (30 minutes per day a minimum of five times per week) (ACSM, 2007: Guidelines for exercise testing and prescription). There is now recognition within exercise psychology literature of immediate improvements in mood state following aerobic exercise, which may bode well for attracting the sedentary population. Despite this there is scant research investigating the effects of perceived exertion on the mood changes associated with exercise. Purpose: This study was designed to examine the effect of rating of perceived exertion (RPE) on the relationship between exercise and mood, in a natural exercise setting. Method: Thirty participants consisting of 24 males and 6 females completed a one mile run, the positive and negative affect scale (PANAS) was administered before and after exercise to give pre and post scores for both positive and negative mood. Rating of perceived exertion was also reported after exercise using Borg’s perceived exertion scale and participants were separated into high, medium and S70 Abstracts Downloaded by [University of Central Lancashire] at 15:03 02 January 2012 low groups based on their (RPE) score. 263 (Time6RPE group) mixed analysis of variance, with repeated measures on the first factor, examined the effects of exercise on positive mood subject to (RPE) group. Results: Overall participants reported significant improvements in both positive and negative mood state in response to exercise; the high (RPE) group demonstrated the greatest improvements in positive mood whereas the moderate (RPE) group demonstrated the greatest improvements in negative mood. Discussion: This study demonstrates that an acute bout of aerobic exercise can facilitate improvements in both positive and negative mood. The findings have implications for the prescription of exercise designed to maximize mood improvement and thus capitalize on the improvement to subsequently improve exercise participation and adherence. Exercise programs may wish to encourage participants to work at a moderate to high level of exertion. Conclusion: The results also have implications for clinical psychologists giving more support for exercise as an effective treatment for depression. Future research should focus on establishing a correlation between people who experience improved mood in response to exercise and subsequent exercise adherence.