The therapeutic effect of the new nonsteroidal aromatase inhibitor fadrozole hydrochloride (4-(5,6,7,8-tetrahydro-imidazo[1,5a]pyridin-5-yl)benzonitrile monohydrochloride, fadrozole, CAS 102676-31-3, CGS16949A) was assessed using a surgically induced endometriosis model in rats. In nontreated rats, the endometrial transplants on the abdominal wall developed large cysts with fluid. In the fadrozole treated group, the cystic volume of the transplants decreased in a dose-dependent manner. The weights of the hemi-uteri were markedly reduced by fadrozole treatment. Fadrozole produced a dose-dependent increase in percentage of vaginal diestrous days. In ovariectomized group, the growth of the transplants was also suppressed, and the weights of the hemi-uteri decreased markedly. Histologically, the signs of growth suppression and/or atrophic changes such as minimization of luminal size, cuboidal piknotic epithelium and/or contraction of stroma were observed in ovariectomized or fadrozole treated groups. The right uterine horns also showed marked atrophic changes as with the transplants. The present study strongly indicates that the new selective aromatase inhibitor fadrozole should be useful for the treatment of endometriosis.