Red blood cell (RBC) properties were proposed to play role in the development of hypertension (HT). This study aimed at investigating the alterations of RBC deformability and aggregation, in various models of HT in rats. The following four models of HT were developed in rats: one kidney-one clip HT (1K-1C HT), two kidney-one clip HT (2K-1C HT), deoxycorticosterone acetate (DOCA) induced HT (15 mg/kg, 2 times/week, sc) and N-nitro L-arginine methyl ester (L-NAME) induced HT (50 mg/kg/day, 10 weeks, ip). The blood samples were obtained from abdominal aorta, under ether anesthesia, after a period of 10 weeks of increased blood pressure. RBC deformability was determined by ektacytometry. RBC aggregation was measured in autologous plasma and 0.5% dextran 500, using a photometric rheoscope. Plasma fibrinogen concentration was determined by Clauss method. The mean blood pressure in all four HT models were about 140 mmHg, on the day of sampling, compared to approximately 110 mmHg in the control group. RBC deformability was found to be significantly decreased in the L-NAME model of HT. RBC aggregation in autologous plasma was significantly higher than control in 2K-1C, L-NAME and DOCA models, DOCA HT model being the most effective in altering the RBC aggregation. Plasma fibrinogen values were found to be higher than control in 2K-1C and L-NAME HT models, but not in DOCA HT. These results confirm that RBC rheological properties might be altered in HT. It can also be suggested that these alternations may not simply be the result of the vascular effects of HT, but may play role in the development of HT, as the alterations in different HT models were not the same, although the length and magnitude of increased blood pressure were similar.