Purpose. The purpose of this study was to use a match-mismatch paradigm to examine three psychological constructs that potentially influence children’s beliefs about participation in an aerobic physical activity: exercise discomfort, self-efficacy, and enjoyment . Methods. Thirty-four middle school females (n =18) and males (n = 16) aged 11-14 years completed the: (1) Exercise Discomfort Index (Children’s OMNI Perceived Exertion (RPE) Scale x Children’s OMNI Perceived Muscle Hurt (RMH) Scale; (2) Exercise Self-Efficacy Scale: Running; and, (3) Physical Activity Enjoyment Scale. Measurements were obtained prior to and following performance of the nationally standardized PACER (Progressive Aerobic Cardiovascular Endurance Run) field test of aerobic fitness. Results. Dependent t-tests for the total (p = .009) and female (p = .042) groups indicated that predicted was greater than actual exercise discomfort; while for the male (p = .057) group predicted and actual exercise discomfort did not differ. Idiographic analysis showed that overpredictors of discomfort reported less time engaged in recreational activity than underpredictors. Exercise self-efficacy was greater (p < .001) pre-, than post-exercise for both male and female children. Exercise enjoyment was the same (p = .400) pre-, and post-exercise for both male and female children. Pearson correlation coefficients for interrelations between exercise discomfort, self-efficacy and enjoyment were not significant with the exception of a significant relation (r = .302) between post-exercise self-efficacy and post-exercise enjoyment. A significant relation was observed between PACER laps completed and pre-, (r = .582) and post- (r = .703) exercise self-efficacy, but not between PACER laps completed and discomfort or enjoyment. Conclusion. Employing a match-mismatch experimental paradigm suggested that exercise discomfort, self-efficacy and enjoyment were psychological constructs that may influence children’s beliefs about an aerobic physical activity. It is possible that one or more of these psychological constructs plays an important role in the initiation and maintenance of aerobic exercise. Such findings can in turn inform physical activity interventions and/or innovative health-fitness components of Physical Education curricula intended to promote cardiovascular health and fitness through regular participation in aerobic physical activity.