Medium dose cyproterone acetate (CPA; 10 mg daily p.o.) was administered to 10 fertile men for 12 weeks. Hormonal measurements and semen analyses were performed before, during (4th and 12th week) and after CPA treatment. At the hypothalamo-pituitary level CPA significantly reduced the hypophyseal storage and synthesis capacity for gonadotropins. Basal LH and FSH concentrations were suppressed by 30% and 40%, respectively; while basal prolactin was elevated by 75%. The episodic fluctuations of peripheral gonadotropins remained unaffected. At the testicular level CPA significantly decreased basal testosterone and dihydrotestosterone concentrations by 70% and 50%, respectively. The pulsatile pattern of androgen secretion was abolished. CPA also inhibited spermatogenesis and motility. No serious clinical side effects were observed. All changes appeared to be completely reversible. It is concluded that medium dose CPA induces progestational/anti-gonadotropic effects at the hypothalamo-pituitary level in addition to its antiandrogenic action at the testicular level.