In neonatal mammals, the drop in oxygen consumption (VO2) during moderate degrees of hypoxia is a manifestation of metabolic depression, and occurs without anaerobic energy compensation. We examined the possibility that embryos also respond to hypoxia with a similar hypometabolic response, by measuring the extent of the O2 debt during post-hypoxic recovery. In chicken embryos at incubation days 11 (E11) and 16 (E16), and hatchlings on the day of hatching (H1), VO2 and carbon dioxide production (VCO2) were measured with an open flow system. The protocol consisted of 1h in air, followed by 40 min in hypoxia (either 17% or 11% O2) and additional 45 min of post-hypoxic recovery. VO2 dropped in hypoxia, more with 11% than 17% O2, and more the younger the animal. During post-hypoxic recovery VO2 returned to, but did not exceed, the pre-hypoxic level, indicating that no O2 debt was contracted during hypoxia. In H1, the changes of VCO2 during hypoxia and post-hypoxia matched those of VO2. Differently, in the embryos, the changes in VCO2 during hypoxia and post-hypoxic recovery were minimal. This phenomenon is explained by changes in the large CO2 stores of the eggs, which buffer the changes in CO2 output of aerobic origin. We conclude that in the chicken embryo and hatchling the energetic shortfall during the hypoxic decrease in VO2 is not compensated by anaerobic energy supply, and represents a phenomenon of metabolic depression.