Previous reports have suggested that plasma norepinephrine levels during exercise, an indicator of sympathetic neurologic activity, may be less in children than in adults. This study investigated plasma norepinephrine values at rest, during two submaximal cycle exercise intensities, and at maximal exercise in 11 boys aged 10 to 12 years and 11 men aged 24 to 35 years. Blood specimens were drawn at average submaximal exercise intensities of 58.7% and 73.0% for the boys and 55.3% and 73.3% VO2max for the men. Weight-relative maximal aerobic power was similar in the two groups. No statistically-significant differences were observed in plasma norepinephrine values at rest or during submaximal and maximal exercise between the boys and men. Maximal levels were 1196 (326 SD) and 1385 (612 SD) pg.ml-1 for the boys and men, respectively (p > 0.05). These findings suggest that sympathetic influences during maximal and submaximal exercise are independent of biological maturation.