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Fifty-five preadolescent boys of European ancestry were given submaximal exercise tests in La Paz, Bolivia (mean altitude of 3600 m). Twenty-nine of the boys were born at high-altitude (HAB) and 26 were born at low-altitude (LAB). V02 and relative work intensity (V02W02max) were significantly lower in HAB boys than in LAB boys, suggesting that the HAB boys were better adapted to hypobaric hypoxia than the LAB boys. After controlling for relative work intensity, there were no significant differences between the groups in their physiological responses to sub­maximal exercise, suggesting that the greater V02 of the LAB boys was not due to a less efficient oxygen transport system. Also, interindividual variability was considerably greater in LAB than HAB boys for many measures, including V02. This may reflect considerable individual differences in the development of adaptive responses to hypobaric hypoxia among LAB boys.