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To test the effects of antiandrogen treatment on the endocrine system 20 normal young men were treated with 750 mg/day flutamide [4-nitro 3'-trifluoromethylisobutyranilide (FLU)] or 100 mg/day cyproterone acetate (CPA) for 14 days followed or preceded by a placebo treatment of the same duration 4 weeks apart. Allocation of both drugs and sequence of active and placebo substance was by random number in a double blind design. On days 1 and 14 two basal blood samples were obtained 15 min apart followed by an iv injection of 25 micrograms GnRH and 10 mg metoclopramide (MTCL). Subsequently blood samples were taken after 25, 45, 60, and 90 min. On day 8 the GnRH/MTCL test was performed after 13 basal blood samples were obtained to establish a baseline for PRL concentrations unbiased by short term stress. After 14 days of FLU treatment, serum testosterone (T) increased significantly [17.4 +/- 1.4 (SE) vs. 26.9 +/- 1.5 nmol/liter, as well as estradiol (E2) concentrations (144 +/- 12 vs. 177 +/- 20 pmol/liter). Dihydrotestosterone increased slightly after 14 days of FLU administration (1.59 +/- 0.14 vs. 1.98 +/- 0.30 nmol/liter). At the same time basal LH concentrations were significantly elevated (5.9 +/- 0.5 vs. 8.6 +/- 0.6 ng/ml), whereas FSH levels were not affected (2.8 +/- 0.6 vs. 2.8 +/- 0.6 ng/ml). No changes in PRL values were detected throughout treatment with FLU. The effect of CPA was quite different. At the end of the treatment phase, serum T levels were reduced considerably (14.6 +/- 1.8 vs. 3.3 +/- 0.4 nmol/liter). Serum E2 decreased similarly (95 +/- 13 vs. 35 +/- 6 pmol/liter). Conversion of T to dihydrotestosterone was also diminished significantly (1.72 +/- 0.33 vs. 1.14 +/- 0.33 nmol/liter). Both gonadotropins were significantly depressed after 14 days of CPA administration (LH, 5.6 +/- 0.7 vs. 2.8 +/- 0.4 ng/ml; FSH, 2.4 +/- 0.3 vs. 1.4 +/- 0.2 ng/ml). Basal PRL levels were not affected, although the response of MTCL was reduced at the end of the treatment phase. Both drugs reduced the response of LH and FSH to GnRH administration.