The object of this study was to evaluate the effect of Bupleuri Radix, aqueous extracts of the root part of Bupleurum falcatum on the 6-n-propyl-2-thiouracil (PTU)-induced rat hypothyroidism. Aqueous extracts of Bupleuri Radix (BR; yield = 11.73%) were administered, once day for 42 days from 2 weeks before start of PTU treatment as an oral dose of 300 and 150 mg/kg (body weight), and hypothyroidism was induced by daily subcutaneous treatment of PTU 10 mg/kg for 28 days. The changes on the body weight, thyroid gland weights, serum thyroid hormone - thyroid stimulating hormone (TSH), triiodothyronine ($T_3$) and thyroxine ($T_4$), serum lipid profiles - total cholesterol, low density lipoprotein (LDL), high density lipoprotein (HDL) and triglyceride, liver antioxidant defense system - lipid peroxidation, $H_2O_2$, superoxide dismutase (SOD) and catalase (CAT), serum aspartate aminotransferase (AST) and alanine aminotransferase (ALT) were observed with histopathology of thyroid glands. Results were compared with $LevoT_4$ 0.5 mg/kg treated rats. As results of PTU treatment, marked decreases of body weights, triglyceride contents, liver CAT activities and changes of serum thyroid hormone levels were observed with increases of serum AST, HDL contents, liver $H_2O_2$ and SOD activities and thyroid gland weight. In addition, marked hyperplasia of follicular cells with decreases of follicular colloid contents and sizes were demonstrated at histopathological inspections. However, these PTU induced hypothyroidism were dose-dependently inhibited by treatment of BR extracts, and BR extracts effectively regulated the hypothyroidism related changes on the antioxidant defense system. The results obtained in this study suggest that BR extracts have favorable effects on the thyroid hormone productions with beneficial effects on the hypothyroidism mediated by the modulatory effects on the antioxidant defense system.