Abstract: This study examined the effect of administration of combined Oral Contraceptive (OC) on blood viscosity and associated hemorheological parameters such as plasma viscosity, Pack Cell Volume (PCV), serum albumin, fibrinogen and total plasma proteins. Female wistar rats aged 7-10 weeks were selected and randomly distributed into two groups, the control group and the OC-treated group, with 20 rats in each group. The rats in both groups were fed with standard rat chow. After two weeks of acclimatization, rats in OC-treated group received OC therapy (containing 1.0 μg ethinyloestradiol and 10.0 μg of norgestrel), while rats in the control group remained on standard rat chow. After 7 weeks of treatment, hemorheological parameters were determined using standard hemorheological techniques described by Dacies and Lewis. Unpaired t-test was performed in all data with the significant level set at p<0.05. There was no significant change in level of fibrinogen, while there was significant increase in blood viscosity, plasma viscosity, PCV, serum albumin concentration and total plasma proteins in OC-treated rats. This finding suggests that increase blood viscosity and plasma viscosity seen in OC therapy is associated with an increase in PCV and serum albumin level.