The weights of prostate and seminal vesicle were decreased by treatment with 3, 6-dimethylpyrazine-2-thiol (3, 6-DMP-SH) but the weights of testes and the plasma testosterone levels were not changed. The recovery of the weight of prostate and seminal vesicles of testosterone-treated orchidectomized rats were inhibited by treatment with 3, 6-DMP-SH. Direct injection of 3, 6-DMP-SH into accessory reproductive organs also induced the decrease in the weight of relevant organs. Administration of oxendrone or chlormazinone along with 3, 6-DMP-SH decreased the weights of accessory reproductive organs more than administration of only one substance. In vitro uptake of testosterone into prostate and seminal vesicle were blocked by the presence of 3, 6-DMP-SH. These results suggest that 3, 6-DMP-SH directly inhibited testosterone uptake into the accessory reproductive organs.