The effect of the traditional herbal medicine, Shakuyaku-Kanzo-To (SK), which contains Shakuyaku (S) and Kanzo (K) in equal amounts, on serum testosterone levels was investigated in androgen-sterilized rats. They were given orally SK [0, 22.5, 45, 90 and 180 mg/kg body weight (b.w.)] and S or K (0, 11.25, 22.5, 40 and 90 mg/kg b.w.) in 2 ml of water daily for 2 weeks. Dose-dependent decreases in free serum testosterone (T) levels were found in the administration of SK. Total serum T levels in the administration of SK and S decreased in a dose-dependent manner. K decreased total serum T levels slightly in 11.5 and 22.5 mg/kg doses but showed the dose-dependent increase in much higher doses, the extent of which was much less than that of the decrease in S. Serum estradiol/T (E2/T) ratios were significantly elevated in 45 to 180 mg/kg doses of SK, 90 mg/kg dose of S and 11.25 to 90 mg/kg doses of K. Serum LH and FSH levels were not changed by SK, S and K. Oophorectomized rats were similarly given SK (0, 90 and 180 mg/kg b.w.) and S or K (the half doses of SK). There were no changes in serum T, LH and FSH levels in all given doses. Thus, one of the mechanisms for SK to lower serum T levels is the direct action on the ovary to stimulate the aromatase activity, resulting in decreasing the T secretion and this is the additive effects of S and K.