Diethylstilbestrol (DES) and its diphosphate (DP) are standard therapies for the prostatic carcinoma (PC). The effects of these compounds and of DES monophosphate (MP) were compared with regard to hormone receptor affinities, uterotrophic properties, inhibition of accessory sex organ weights, and tumor-inhibiting properties in hormone-dependent mammary tumor models in vitro and in vivo as well as Noble Nb-PC models. The estrogenic activity decreased in the order DES > MP > DP. The MCF-7 breast cancer cell line was inhibited by DP and to a somewhat lesser extent by DES and MP. In the MXT mammary tumor in vivo, all three compounds exerted a stimulatory effect at low doses, but were strongly inhibitory at higher doses. The accessory sex organ weights of intact male mice and rats were strongly reduced by DES, MP, or DP. In the hormone-sensitive Nb-H and in the Nb-R PC models of the rat, DES as well as DP caused a significantly better inhibition of tumor growth than did castration. The slightly inhibitory effect of castration was strongly potentiated by simultaneous application of DES. Therefore, in addition to the endocrine effects of DES and DP, a direct PC-inhibiting effect is obvious.