Large dose of Yang-restoring herb medicines. (Radix Codonopsis Pilosulae, Astragalus membranaceus, Radix Aconiti Praeparata, Epimedium brevicornum, Cortex Cinnamomi and Herba Cistanchis) may exert an unfavorable effect on normal rats, i. e. natural weight gain reduced (P less than 0.01), serum T3 decreased (P less than 0.05), rT3, TRH levels raised (P less than 0.01) and TSH showed a raising tendency. Lower T3 and higher rT3 levels may be the effects of Yang-restoring herb medicines on the peripheral metabolism of thyroid hormones, i. e. more thyroxine was degraded to rT3 with little biological effect and less was transformed to T3 with strong hormonal effect. This unfavorable effect, however, can be avoided by Forward-Backward method. It was advised that large dose of Yang-restoring herb medicines could not be given to the organisms without symptoms of Yang-deficiency. If it were tried to do so, Forward-Backward method might be recommended.