The importance of ashwagandha root extract in the regulation of thyroid function with special reference to type-I iodothyronine 5'-monodeiodinase activity in mice liver has been investigated. Although the root extract (1.4 g kg(-1)) administered daily for 20 days by gastric intubation increased serum 3,3',5-triiodothyronine (T3) and tetraiodothyronine (T4) concentrations and hepatic glucose-6-phosphatase activity, hepatic iodothyronine 5'-monodeiodinase activity did not change significantly. Furthermore, ashwagandha root extract significantly reduced hepatic lipid peroxidation, whereas the activity of antioxidant enzymes such as superoxide dismutase and catalase were increased. These findings reveal that the ashwagandha root extract stimulates thyroidal activity and also enhances the antiperoxidation of hepatic tissue.