In animal experiments blood samples are often taken from various parts of the circulation. Although several variables including blood gas parameters are known to alter comparing arterial to venous system, arterio-venous (A-V) differences of blood micro-rheological variables (erythrocyte deformability and aggregation) tested by ektacytometry and aggregometry are not completely known in laboratory rats. In 12 outbred rats we investigated red blood cell deformability (RheoScan-D200 slit-flow ektacytometer), red blood cell aggregation (Myrenne MA-1 erythrocyte aggregometer), hematological variables (Sysmex F-800 microcell counter), blood pH and blood gases (ABL555 Radiometer Copenhagen) in blood samples taken parallel from the abdominal aorta and from the caudal caval vein. Blood pH did not differ, blood gas partial tensions showed physiological A-V differences, as it was expected. White blood cell count, red blood cell count and hematocrit were significantly higher in samples from the caval vein. Erythrocyte aggregation values (at 3 1/s shear rate) were significantly higher in samples taken from the abdominal aorta. Erythrocyte deformability (elongation index) did not show obvious A-V differences. Arterio-venous hemorheological differences -mostly of erythrocyte aggregation- can be found in rats, thus, the standardization of the studies and planning appropriate control measurements are necessary for safe evaluation of the obtained results.