Hemorheological values vary widely among the animal species. To understand the structure-function relationships of red blood cells (RBC) together with associated physiological mechanisms, comparative studies are still a classical approach. Some animal species show extraordinary values of blood viscosity and RBC indices which would be pathologic for man. These differences can reflect an adaptation process to a specific environment or way of life. Different species also use different mechanisms to maintain blood flow, and in some cases these differences might indicate which variables limit the demand for oxygen delivery in a species or under certain circumstances. Nevertheless, when comparing values to man, it should be kept in mind that every hemorheologic “disturbance” in a healthy animal reflects a physiologic circumstance. It is necessary to know that the hemorheological profile of an animal cannot be judged by a single rheological value, but must be considered as part of the cardiovascular system in which the blood is flowing. That is, the hemorheological profile of an animal species is a conglomerate of properties which has to be considered in evaluating its cardiovascular relevance in a species-specific manner. These speciesspecific differences look chaotic at first sight, and no clear guiding principle has been found to combine these profiles to a universal “logic” at present. Therefore changes in parameters during disease, after interventions, or during environmental or associated changes need to be related to their species-specific reference values; describing an animal's clinical or physiological condition using comparisons to values from other species is most likely not a valid approach.