Red blood cell (RBC) deformability values resulted from ektacytometry tests can be influenced by the viscosity of the medium in which the RBCs are suspended for measurement. To determine the power of measurements using various viscosity media in this study we used normal and heat treated RBCs from laboratory rats and beagle dogs. A RheoScan-D200 slit-flow ektacytometer was used to measure RBC deformability. Blood samples were taken from female Sprague-Dawley rats and inbred beagle dogs. Before and after heat treatment of RBC suspensions ektacytomctry tests were performed using PVP solutions at viscosity of 15, 20 and 30 mPa.s. Heat treatment caused impaired RBC deformability in both species and in every PVP solution. The difference between normal and heat treated RBCs was the highest in rats, while in dogs the magnitude of change was smaller, however being significant. Well comparable and stable results were found using 30 mPa.s media. The sensitivity of RBCs for heat treatment seems to be higher in rats. The suspending PVP medium at 30 mPa.s is recommended for testing RBC deformability by ektacytometry in laboratory rats and dogs, too, because this media resulted in the most stable data when comparing normal and rigid cells.