The physiological significance of the position and shape of the oxygen equilibrium curve (OEC) of horse hemoglobin (Hb) is considered from the viewpoint of oxygen (O₂) transport efficiency and the effectiveness of the Bohr effect. In horse fetal and maternal bloods, their physiological O₂ affinities are nearly optimized with respect to the effectiveness of the Bohr shift occurring at the O₂ release site, when it is measured by the change in O₂ saturation per unit change in P₅₀. With relatively low cooperativity (n=2.69) of horse Hb under physiological conditions, the effectiveness of the Bohr shift for fetal blood at O₂ uptake site and maternal blood at O₂ release site is high. These facts imply that the position and the cooperativity of horse Hb OEC are optimized to receive maximal benefit from the double Bohr shift. Before exercise, the position of the OEC for adult mares is nearly optimized for the effectiveness of the Bohr shift occurring at the O₂ release site, whereas, at maximal exercise, the position of the OEC tends to become advantageous for O₂ transport efficiency.