There is little data on blood viscosity in different animals. Therefore a comparison was performed between five species by an in-vitro method using oscillatory flow. At shear rates from 1 to 150/sec the viscous and the elastic component of the complex blood viscosity was highest in pig, followed by horse, dog, ox, and sheep. Plasma viscosity and plasma density were similar in dog and sheep and were higher in ox. The differences in whole blood viscosity were obviously related to the variable interspecies hematocrit, and may also be influenced by different aggregation tendencies. Aggregation in ox was low despite a high plasma fibrinogen. It seems that erythrocytes in sheep and ox remain separately suspended in a relative dense plasma. This may indicate that formation of microthrombi is less likely to occur in ox and sheep than in pig, horse, and dog.