Hypoxia inhibits avian embryonic development, as well as increases embryonic mortality. However, the key organ systems affected by hypoxia, and their critical windows for development, are poorly understood. Consequently, chicken embryos were continuously exposed to 3 levels of oxygen (21, 15, or 13% O(2)) throughout d 0 to 10, d 11 to 18, or d 0 to 18 of incubation, followed by morphometric and blood physiological measurements. Hypoxia occurring early during incubation (d 0 to 10) had larger effects on embryonic mortality and organ growth than hypoxia occurring at later stages (d 10 to 18). Growth of the heart and chorioallantoic membrane was stimulated by chronic hypoxia, whereas the lung, brain, eye, liver, stomach, beak, and toes showed no disruption. Sustained hypoxia from the beginning of incubation decreased blood hemoglobin, hematocrit, and red blood cell concentration of embryos at d 10, but the values among hypoxic and normoxic groups were not significantly different at d 18. Blood partial pressure of O(2) and partial pressure of CO(2) were dependent upon incubation O(2) level at a given day of development. These results indicated that either modest hypoxia (15% O(2)) throughout development, or hypoxia at any level during the late stages (d 11 to 18), increased the heart and chorioallantoic membrane weight, which partly compensated for the detrimental effects of hypoxia on embryonic development. We conclude that the first half of embryonic development contained the critical windows for the detrimental effects of hypoxia, and the second half contained the critical windows for the compensatory response of hypoxia in key organs.