In 36 men with prostatic cancer, the following findings were obtained: intravenous administration of 12.0 g diethylstilboestrol diphosphate (DSDP) induced a relatively slight decrease of the LH plasma level from 22.7 +/- 11.8 to 7.7 +/- 3.6 mIU/ml (34%), whereas the total testosterone plasma level decreased from 435.3 +/- 187.8 to 29.9 +/- 16.4 ng/100 ml (6.7%) suggesting a direct inhibitory effect of the oestrogen on testicular testosterone secretion. The apparently free, biologically active testosterone plasma level even decreased from 6.2 +/- 3.7 to 0.21 +/- 0.16 ng/100 ml (3.4%), due to the oestrogen-induced increase of the concentration of testosterone-binding beta-globulin (from 9.6 +/- 4.4 to 20.6 +/- 10.7-10(-8) M). 3--7 days after additional orchidectomy plus subcutaneous implantation of 100 mg dienoestrol diacetate a further decrease of the apparently free testosterone plasma level from 0.21 +/- 0.16 to 0.14 +/- 0.07 ng/100 ml was found. In contrast, 6 weeks after orchidectomy without oestrogen implantation a significant increase of th- apparently free testosterone plasma level -rom 0.21 %/- 0.16 to 0.34 +/- 0.15 ng/100 ml was observed (p less than 0.01). In view of these findings the biologically active free testosterone plasma level appears to be even more suppressed by intravenous administration of high DSDP than by orchidectomy. The most effective suppression of apparently free testosterone was achieved, however, by oestrogen treatment combined with orchidectomy.