[en] The purpose of this study was to determine the effect of regulating factors on the oxygen equilibrium curve (OEC) under standard conditions and then to calculate the oxygen extraction between arterial and jugular venous blood in healthy Standardbred horses. The results were compared to those previously obtained in humans and cattle, using the same experimental method. The partial oxygen pressure at 50% saturation of haemoglobin, measured under standard conditions (standard P50), was 24.8 +/- 2.0 (SD of mean) mm Hg. This value was similar to the cattle standard P50 (25.0 +/- 1.4 mm Hg, SD of mean) but lower than the human standard P50 (26.6 +/- 1.2 mmHg, SD of mean) previously reported using the same experimental method. The effects of regulating factors on the standard OEC were also determined, and a major effect of pH and temperature was noted. In contrast, partial carbon dioxide pressure played only a minor role in horses, compared to cattle and humans. No significant correlation was found between phosphate and chloride concentrations and standard P50. (c) 2004 Elsevier Ltd. All rights reserved.