Oxygen affinity and Bohr effect have been measured in the blood of four bird species (chicken, pigeon, Japanese quail and sparrow). Under physiological conditions (41 C, pH 7.5, PCO2 approximately 35 Torr) the oxygen half saturation pressure P50 are 50 Torr for the chickens, 38 Torr for the pigeon, 43 Torr for the Japanese quail and 44 Torr for the sparrow. Values for the physiological Bohr factors deltalogP50/deltapH are similar to those obtained in mammalian blood ranging from -0.57 in chicken blood to -0.42 for pigeon blood. A direct effect of CO2 on the oxygen affinity of chicken blood could not be observed, in agreement with results in hemoglobin solutions. However, at a constant pHpl of 7.5 the intracellular pH rise from 7.22 at 39 Torr PCO2 to 7.32 at PCO2 less than 1 Torr. This leads to a decrease of P50 at pHpl 7.5 from 52 to 46 Torr. A convincing correlation between body weight and P50 could not be established, even when including data from other authors. It is concluded that aside from metabolic rate, behavioural as well as environmental factors have influenced whole blood oxygen affinity during evolution.