We have investigated the influence of hypoxia (13.5% O2) on the oxygen affinity, hemoglobin pattern, and blood volume of chicken embryos. Data were collected between 4 and 9 days of incubation. It was found that the transition from embryonic to adult hemoglobin starts earlier in hypoxic embryos, due to a premature appearance of definitive red cells in the circulation. Blood oxygen capacity and total blood volume (related to embryonic weight) were not different from the controls. However, hypoxic embryos weighing more than 0.1-0.2 g have a significantly increased oxygen affinity (and decreased red cell adenosine 5'-triphosphate concentration). The higher O2 affinity partly compensates for the adverse effects of the lowered environmental PO2. In hypoxic embryos younger than 6 days, the Bohr effect is drastically increased, which allows a better oxygen extraction. Thus, the chicken embryo seems to be able to develop adaptive changes of blood O2 transport during hypoxia. The results support the view that the particular ontogenetic pattern of O2 affinity changes seen in avian or mammalian embryos arose from the need to adapt to the steadily changing conditions for O2 uptake.