A comparative study has been made of the blood rheology, and its component factors, in horse, sheep, cattle, goat, camel, pig, dog, rabbit and man. The erythrocyte flexibility of horse red cells is high relative to man, that of pig, dog, camel and rabbit comparable, but less flexible, and sheep, cattle and goat relatively inflexible. The erythrocyte flexibility of horse, sheep, cattle and goats does not vary with the plasma fibrinogen level, as occurs with human and rabbit cells. Washing erythrocytes and then suspending them in isotonic saline makes the erythrocytes of all species relatively inflexible. There is a factor in horse plasma, which is not fibrinogen, that makes horse and human erythrocytes suspended in it very flexible. The blood viscosity of all species is comparable at high shear rates (230 s-1) due to the shape of the cells compensating for their flexibility. The variations of blood viscosity at low shear rates (11.5 s-1) were also found to depend on the erythrocyte flexibility, and only influenced indirectly by the fibrinogen concentration. There is no significant effect of temperature on the erythrocyte flexibility of horse, sheep, cattle, goat and a small number of human subjects. This is reflected in the way the viscosity of these bloods varies with temperature.