The effects of Ocimum sanctum leaf extract on the changes in the concentrations of serum triiodothyronine (T3), thyroxine (T4) and serum cholesterol; in the activities of hepatic glucose-6-phosphatase (G-6-P), superoxide dismutase (SOD) and catalase (CAT); hepatic lipid peroxidation (LPO) and on the changes in the weight of the sex organs were investigated. While the plant extract at the dose of 0.5 g kg-1 body wt. for 15 days significantly decreased serum T4 concentrations, hepatic LPO and G-6-P activity, the activities of endogenous antioxidant enzymes, SOD and CAT were increased by the drug. However, no marked changes were observed in serum T3 level, T3/T4 ratio and in the concentration of serum cholesterol. It appears that Ocimum sanctum leaf extract is antithyroidic as well as antioxidative in nature.