The object of this study was to evaluate the effect of Lonicerae Flos, aqueous extracts of the dried flower bud part of Lonicera japonica on the 6-n-propyl-2-thiouracil (PTU)-induced rat hypothyroidism. Aqueous extracts of Lonicerae Flos (LF yield = 23.80%) were administered, once day for 42 days from 2 weeks before start of PTU treatment as an oral dose of 500 and 250 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), tri-iodothyronine ($T_3$) and thyroxine ($T_4$), serum lipid profiles - total cholesterol, low density lipoprotein (LDL), high density lipoprotein (HDL) and triglyceride were observed with liver antioxidant defense system - lipid peroxidation, $H_2O_2$, superoxide dismutase (SOD) and catalase (CAT) and serum asparte aminotransferase (AST) and alanine aminotransferase (ALT) analysis. Results were compared with Levothyroxine ($LT_4$) 0.5 mg/kg treated rats. As results of PTU treatment, marked decreases of body weights, serum thyroid hormone levels and triglyceride contents, liver $H_2O_2_3$ and SOD activities were observed with increases of serum AST and HDL contents, liver CAT activities, thyroid gland weight. However, these PTU induced hypothyroidism were dose-dependently inhibited by treatment of LF extracts, and LF extracts effectively regulated the hypothyroidism related changes on the antioxidant defense system. The results obtained in this study suggest that LF 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.