Sixty-six female Sprague-Dawley (SD) rats with 7,12-dimethylbenz [α] anthracene(DMBA)-induced rat mammary cancer were divided into five groups: tamoxifen (TAM), medoroxyprogesterone acetate (MPA), TAM + MPA, ovariectomy (Ovex), control (no treatment). An antitumor effect was shown in each treated group. Thirty-six (72%) out of 50 treated animals responded to the first endocrine therapy. Moreover, tumors disappeared completely from 21 out of the 36 animals, and no new tumors were seen until the 12th week. The facts suggest experimental hormone, when compared to clinical, therapy in DMBA-induced tumors to have a higher response rate. A total of 14 out of 50 tumors failed to respond to the first treatment (resistant tumor). There was no significant difference in the proportion of resistant tumors among the four treated groups. When other endocrne therapies were tried out of resistant tumors, seven out of the 14 responded, the resistant tumors in the TAM group responding significantly well to the other endocrine therapies compared to those in the MPA group (P< 0.05). These results suggest the possibility of resistant tumors responding to different types of endocrine therapy.