The effect of Synacthen (beta1-24-corticotrophin) on plasma testosterone and 4-androstene-3, 17-dione concentrations in untreated patients with prostatic carcinoma, and in patients receiving endocrine therapy is described. An established specific radioimmunoassay was used for the measurement of testosterone, and a radioimmunoassay for 3-androstene-3,17-dione using thin layer chromatography has been developed. Administration of Synacthen resulted in a fall in testosterone in untreated patients, but a rise in 4-androstene-3,17-dione was observed. The plasma concentration of testosterone in all treated patients increased after administration of Synacthen. An increased concentration of plasma 4-androstene-3,17-dione was also observed in these treated patients after Synacthen, but the magnitude of the response was not significantly different from that of untreated patients. The work provides further evidence that in the patient being treated with oestrogen for carcinoma of the prostate a rise in plasma testosterone concentration will result from an increased secretion of ACTH.