Treatment effects of RU 23908 antiandrogen (Anandron®) and estrogen in low doses on hormone‐dependent rat prostatic adenocarcinoma (R3327‐H) were investigated. Tumorbearing Copenhagen rats were treated for 6 weeks with 8 μg Anandron and 1 μg estradiol‐17β every two days. Reduction and counteraction of androgen synthesis and action was established by an observed decline in serum testosterone level and by changes in both histology and weight of androgen target organs. Prostate tumor growth rate was significantly retarded in rats treated with Anandron/Estradiol combination compared to untreated intact control and was equal to the slow growth rate in castrate tumor‐bearing animals. Tumor histology changes during treatment correlated with the observed growth rate retardation. Areas of necrosis, metaplasia. and acellularity were more frequently observed in tumors of Anandron/Estradiol‐treated compared to castrated rats. These results suggest that low doses of Anandron and estrogen can effectively be combined as a complete androgen counteracting therapy for hormone‐dependent prostatic carcinoma with minimal undesired side effects.