Relative importance of Bacopa monnieri (200 mg/kg), Aegle marmelos (1.00 g/kg) and Aloe vera (125 mg/kg) leaf extracts in the regulation of thyroid hormone concentrations in male mice was investigated. While serum levels of both T(3) and T(4) were inhibited by A. vera, A. marmelos extract could decrease only T(3) concentration. On the other hand, T(4) concentration was increased by B. monnieri extract suggesting its thyroid-stimulating role. When the relative potency of each plant extract was calculated in terms of percent increase or decrease in thyroid hormones, as compared to the control value, the decrease in T(3) concentration by A. marmelos was about 62% indicating its possible use in the regulation of hyperthyroidism. B. monnieri could increase T(4) concentration by 41% without enhancing hepatic lipid peroxidation (LPO) suggesting that it can be used as a thyroid-stimulating drug. In fact, hepatic LPO was decreased and superoxide dismutase (SOD) and catalase (CAT) activities were increased by B. monnieri and A. marmelos leaf extracts showing their antiperoxidative role. It is thus suggested that A. marmelos and A. vera may be used in the regulation of hyperthyroidism, while B. monnieri in hypothyroidism.