According to data obtained previously on the blood gas tensions and the oxygen dissociation curve of chicken embryos, the arteriovenous oxygen saturation difference in the allantoic circulation has been conjectured fairly large. In order to confirm this conjecture as well as to check the validity of the in vitro dissociation curve, both the blood oxygen capacity and content in allantoic artery and vein were measured. The in vivo O2 saturation measured here resulted in a similar value to that estimated from the dissociation curve. The O2 content in allantoic vein ranges from about 7 to 11.5 vol% during the 10th to the 18th days of incubation and that in artery is pronouncedly low in a range of 1 to 2.5 vol%, suggesting that the blood flow rate through the body tissues is fairly larger than that through the gas exchange capillary plexus. Then, the distribution of blood flow was estimated from the analyzed data based on a model of blood circulation and some assumptions. In connection with this estimation, the diffusing capacity for deoxygenation in the tissues was speculated to be much larger than that for oxygenation in the chorioallantoic capillaries.