Effects of hypothyroid on hemorheology of patients had widely attracted the attention of researchers during last decade. The present study has been planned with the purpose to determine the effects of experimental hypothyroidism on hemorheological parameters and fibrinogen concentration. To induce experimental hypothyroid methimazole (75 mg/100 g) was added to the fodder of an experimental group rats for 20 d. After experimental duration, plasma and blood viscosity, hernatocrit (Hct), hemoglobin, erythrocyte rigidity index, and plasma fibrinogen concentration values of both the control and the experimental group animals were determined and evaluated. The serum T-3 and T-4 levels of the experimental group were found lower (p < 0.001) but TSH level higher (p < 0.001) than that of the control group. Plasma viscosity and fibrinogen concentration of hypothyroid group were found significantly higher than controls (p < 0.01). Hematocrit and hemoglobin values were also found lower in the experimental group than the control group animals (p < 0.01). However, there was no significant difference found in blood viscosity at the original Hct value but there was a significant increase at standard Hct value (p < 0.01). There was also no change in erythrocyte rigidity index between control and experimental groups. According to these results it may be said that in hypothyroidism, increased fibrinogen concentration may alter the rheological structure of blood by inducing increase in plasma viscosity.