Effects of chronic hypobaric hypoxia on blood O 2 transport were studied using two groups of domestic pigeons (Columba livia). High-altitude (HA) birds were acclimated for 6 wk at 7 km simulated altitude (308 Torr, 25Β°C); sea-level (SL) pigeons were maintained for 5-6 wk at 758 Torr (25Β°C). Whole blood O 2 equilibrium curves (O 2 ECs) were generated for HA and SL pigeons at 37Β°C and 41Β°C using thin film techniques. At pH 7.50 and 41Β°C, half-saturation PO 2 s (P 50 ) for HA and SL birds were 32.9 Β± 1.4 (6); (mean Β± S.E.M. [N]) and 35.6 Β± 0.6 (6) Torr, respectively. At pH 7.56 and 37Β°C, the corresponding P 50 s were 26.5 Β± 1.8 (4) and 29.0 Β± 0.6 (5) Torr, respectively. HA and SL pigeons exhibited similar Hb-O 2 temperature coefficients (βH = -9.8 to -10.6 kcal/mole O 2 ) and CO 2 (Bohr slopes (βlog P 50 /βpH = -.43 to -.44 at 37Β°C; -.41 at 41Β°C). Hill's n increased with increasing saturation for both animal groups at 37Β°C and 41Β°C. Hypoxic acclimation elicited a significant increase in O 2 capacity; %Hct and [Hb] increased by 38% and 43%, respectively. Chronic hypoxia had no effect on isohemoglobin composition; both groups exhibited two isoHb fractions in a 9:1 molar ratio. At P a O 2 = 30 Torr, calculated in vivo C a O 2 for HA (39.2Β°C, pH 7.53) and SL (41Β°C, pH 7.50) birds were 14.2 vol % and 7.6 vol %, respectively. Results suggest that altitude tolerance of pigeons is enhanced by the increased blood O 2 capacity and the effect of hypoxiainduced hypothermia on Hb-O 2 affinity.