ABSTRACTPast studies have found that total‐body O2extraction during hypoxia was less in 1‐wk‐old lambs than in older animals. It was proposed that reduced O2extraction was secondary to suppression of growth‐related oxygen consumption (&OV0312;o2) in tissues such as skeletal muscle, bone, kidney, and skin, rather than a defect in peripheral O2use. To determine the capacity of immature skeletal muscle to extract O2, we isolated the hind limb circulation of eight ketamine‐anesthetized, 7‐ to 18‐d‐old lambs exposed to stagnant hypoxia by inflation of a right atrial balloon catheter. Femoral arterial and venous Po2, Pco2, pH, Hb concentration, O2saturation, and femoral arterial blood flow (Q) were measured and hind limb O2delivery (Do2), extraction ratio, and &OV0312;o2calculated. Individual critical levels of Do2below which Vo2was dependent on O2supply were determined by dual‐line best‐fit regression analysis, In six of eight animals, &OV0312;o2was clearly independent of supply until Do2reached critically low levels. However, O2extraction during extreme hypoxia appeared submaximal (baseline O2extraction ratio, 0.22 ± 0.06; at critical levels of Do2, 0.51 ± 0.11; at the lowest level of Q, 0.64± 0.15). When 2,4‐dinitrophenol, an uncoupler of oxidative phosphorylation, was administered to four additional lambs exposed to stagnant hypoxia, O2extraction below critical levels of Do2increased from 0.48 ± 0.15 to 0.79 ± 0.10 (p< 0.001, unpairedttest). These data suggest that initial limitations in O2extraction were a result of the suspension of O2‐consuming processes, not an irreversible defect in peripheral O2use. (Pediatr Res28: 101‐105, 1990)AbbreviationsDNP, 2,4‐dinitrophenolDo2, O2deliveryDo2crit, critical level of Do2Q, blood flowVo2, O2consumptionP50, Po2at which Hb was 50% saturatedANOVA, analysis of variance[Hb], Hb concentrationpHa, arterial pH