Field tests of running and laboratory tests were performed in La Paz [high altitude (HA), 3700 m] and in Clermont-Ferrand [low altitude (LA), 300 m] to investigate their validity at HA. Prepubertal boys of mean ages 10.6 years (HA1, n = 16; LA1, n = 28) and pubertal boys of 13.7 years (HA2, n = 12; LA2, n = 41) took part in the study. All the boys performed a 30-m sprint (V30m), a 30-s shuttle run (V30s) and a progressive shuttle run test until their maximal aerobic velocity (VmaxSRT). Maximal oxygen consumption was extrapolated from the last test (VO2maxSRT). In the laboratory, the boys performed a force-velocity test (Pmax), a Wingate test (PWing) and a graded test to measure maximal oxygen consumption (VO2maxB; direct method) on a cycle ergometer. At similar ages, there was no significant difference between HA and LA boys for V30m and Pmax. The V30s of HA boys was 3%-4% lower than those of LA boys (P < 0.05); there was no significant difference for PWing. Significant relationships were observed at both altitudes between Pmax (watts per kilogram) and V30m (HA: r = 0.76; LA: r = 0.84) and between PWing and V30s (HA: r = 0.67; LA: r = 0.77); the slopes and the origins were the same at HA and LA. The VO2max, VmasSRT and VO2maxB were lower by 9%, 12% and 20%, respectively, at HA than at LA (P < 0.05). However, the relationships between VO2maxB and VO2maxSRT (litres per minute) at HA (r = 0.88) and at LA (r = 0.93) were identical.(ABSTRACT TRUNCATED AT 250 WORDS)