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Maximal exercise tests were given to 67 boys of European ancestry and between the ages of 8.8 and 13.1 years in La Paz, Bolivia (mean altitude of 3600m). Thirty-four of the boys were born at high altitude while the remaining 33 were born at low altitude. The boys born at high altitude were significantly fatter than the low-altitude born boys after controlling for age and had larger chest circumferences after controlling for stature. Otherwise the samples were morphologically similar. Also, the statures and weights of the boys in both groups were similar to those of normal U.S. boys, suggesting that their growth may not have been affected by hypoxia to any great extent. Although most measures of maximal work performance, including maximal aerobic power (V02max), did not differ significantly between the samples, maximal work output was significantly greater in the high- altitude born boys than in the low-altitude born boys. Length of residence at high altitude and maximal work output were positively related in the low-altitude born boys but no other relationships were found between maximal work performance and length of exposure to hypobaric hypoxia. The considerable individual variability in the responses of these boys to maximal exercise may have masked these relationships, however, and longitudinal studies may be needed to reveal developmental adaptations to hypoxia.