The purpose of this work was to determine the whole Oxygen Equilibrium Curve (OEC) in sheep in order to give reference values and to compare the OEC obtained in sheep to those previously obtained in man and cattle, using the same methodology.

Venous blood samples were collected in healthy adult sheep (n = 12). Oxyhemoglobin equilibrium curve (OEC) was traced in standard conditions (pH: 7.4; PCO₂: 40 mm Hg; temperature: 37°C) using a dynamic tonometric method. Partial oxygen pressures at various saturation levels were as follows:

>SO2 (%)	PO2 (mm Hg) Sheep (n = 12)	PO2 (mm Hg) Cattle (n = 28)	PO2 (mm Hg) Humans (n = 68)
10	12.8 ±  2.5	10.6 ±  1.1*∇	10.2 ±  1.1**
20	19.5 ± 2.3	15.3 ±  1.0***∇	14.9  ±  1.0***
30	25.0 ± 2.7	18.8 ±  1.0***	18.9  ±  1.0***
40	30.0 ± 3.8	22.1 ±  1.1***∇	22.7  ±  1.1***
50	35.3 ± 5.0	25.4 ±  1.3***∇∇∇	26.6  ±  1.3***
60	41.2 ± 6.3	29.3 ±  1.5***∇∇∇	31.1  ±  1.5***
70	48.2 ± 7.3	34.1 ±  1.7***∇∇∇	36.7  ±  1.8***
80	58.2 ± 7.6	40.8 ±  2.1***∇∇∇	44.6  ±  2.3***
90	78.7 ± 6.9	54.0 ±  3.3***∇∇∇	59.6  ±  4.2***

Oxygen affinity of the blood of the sheep is lower than the affinity of the man and the cattle's blood, oxygen affinity being higher in cattle than in humans. Higher P50 value recorded in sheep (compared to human and cattle) could be in favour of tissue oxygenation.