The effect of calusterone (7beta,17alpha-dimethyltestosterone) on rat mammary DMBA-induced adenocarcinoma was studied both in vivo and in organ culture. In vivo all 8 tumours with a diameter of less than 30 mm regressed following calusterone injection (10 mg/day for 2-3 weeks). In organ culture calusterone (20 mug/ml medium) inhibited the synthesis of DNA and RNA in all 7 cases examined. Testosterone also inhibited the synthesis of DNA and RNA in organ culture in 12 out of 14 and 10 out of 14 tumours respectively. Oestradiol-17beta on the other hand had no effect on DNA and RNA synthesis in organ culture although 70% of the tumours examined were ovarian dependent, i.e. regressed following castration. This could be explained by the direct effect of calusterone on rat adenocarcinoma compared with the indirect effect of oestradiol-17beta which probably exerts its action by activating the secretion of prolactin which acts on the tumour.