We asked to what extent sustained hypoxia during embryonic growth might interfere with the normal development of thermogenesis. White Leghorn chicken eggs were incubated at 38°C either in normoxia (Nx, 21% O₂) or in hypoxia (Hx, 15% O₂, from embryonic day E5 until hatching). The Hx embryos had lower body weight (W) throughout incubation, and hatching was delayed by about 10 hours. For both groups, all measurements were conducted in normoxia. At embryonic day E11, the static temperature- oxygen consumption (ambient T-V_O2 ) curve was typically ectothermic (Q₁₀ = 1.92-1.94), and similar between Nx and Hx. Toward the end of incubation (E20), the Q₁₀ averaged 1.41 (±0.06) in Nx and 1.79 (±0.08) in Hx (P<0.005), indicating that the onset of the thermogenic response in Hx lagged behind Nx. In the 1-day old hatchlings (H1) body weight did not significantly differ between Nx and Hx. At H1, the T-V_O2 curves were endothermic-type, and more so in the older (>8hrs old) than in the newly hatched (<8 hrs old) chicks, whether examined statically or dynamically as function of time. In either case, the thermogenic responses of Hx were lower than those of Nx. In a 43-31°C thermocline, the preferred T of the Hx hatchlings was around 37.3°C, and similar to Nx, suggesting a similar setpoint for thermoregulation. We conclude that hypoxic incubation blunted the development of thermogenesis. This could be interpreted as an example of epigenetic regulation, where an environmental perturbation during early development alters the phenotypic expression of a regulatory system.