SUMMARY: Oestradiol-17β, diethylstilboestrol (DES), dl-dihydrodibutylstilboestrol (dl-DHBS) and meso-dihydrodibutylstilboestrol (meso-DHBS) were injected intramuscularly into male Sprague-Dawley rats in a daily dose of 100 μg for a period of 10 days. Oestradiol-17β and DES decreased the weight of the prostate and seminal vesicles to the same extent, whereas meso-DHBS was less effective. dl-DHBS was almost inactive. Only oestradiol-17β and DES caused a decrease in the weight of the testes. The adrenal glands increased in weight after administration of either oestradiol-17β, DES or meso-DHBS. Four hormones in the plasma were measured: testosterone, androstenedi-one, prolactin and interstitial cell-stimulating hormone (ICSH). DES decreased the plasma concentration of both ICSH and testosterone. Oestradiol-17β and meso-DHBS administration resulted in a lowering of the plasma testosterone concentration with no effect on ICSH. Oestradiol-17β, DES and meso-DHBS markedly increased plasma prolactin concentrations. dl-DHBS appeared to have little biological effect causing only very small changes in all the parameters investigated.