The flow properties of blood and its components vary widely throughout the animal kingdom. Even if nucleated avian and reptile red blood cells (RBC) are excluded from the analysis, RBC exhibit different rheological behavior among mammalian species. Both RBC aggregation and cellular deformability have been reported to vary among species, including placental mammals, marsupials, terrestrial and aquatic mammals. Although the relationships between blood flow behavior and species-specific characteristics have not been systematically investigated, studies to date allow recognition of interesting patterns, especially for RBC properties. These properties do not correlate with simple cellular parameters (e.g. mean cell volume), but more detailed analysis of RBC structure may reveal cellular aspects (e.g. surface charge density) that can be related to rheologic behavior. It has been postulated that the athletic capacity of mammalian species may predict the aggregation behavior of their RBC, but this hypothesis has not been supported by data from a wide range of athletic and sedentary species. Aquatic mammals also exhibit a very interesting diversity of hemorheological properties, which again are not yet easily related to specific circulatory adaptations. Data from current comparative studies suggest that a better understanding of the relations between specific hemorheological properties and specific hemodynamic adaptations in a variety of species should contribute to a better understanding of circulatory behavior; future studies are thus clearly indicated.