Seventeen children, ages 8-14 years, were compared to an adult group (n=19) in their ability to execute prescriptive exercise using the Borg 6-20 rating of perceived exertion (RPE) scale. Subjects cycled at a controlled pace (setting resistance) for 2 minutes or traveled one lap of a 400-meter track at four RPE levels: 7, 10, 13, and 16. Results indicated that for pace-controlled cycling, children were similar to adults, reproducing four incremental intensities. Adults were closer to the laboratory determined criterion than the children were. For the self-paced walk/run tasks, both groups overexerted compared to the criterion. Children could only discriminate RPE 7 from the other levels. Based on these observations, it is recommended that exercise prescriptions for children using self-monitored intensity be preceded by adequate practice or be adjusted to account for different modes of activity.