Six patients with advanced prostatic cancer were treated with a potent GnRH agonist analog (buserelin, Hoechst; 600 ng, intranasally, three times a day) for 6 months. The first 2-6 days of treatment were associated with a 50% elevation (P < 0.01) in serum testosterone (T) and a simultaneous elevation of 20% (P < 0.01) in serum prostate-specific acid phosphatase (PAP). Serum T fell to the castrate range (<1 nmol/liter) in all patients in 2-3 weeks, and PAP decreased concomitantly in five of the six patients. Serum LH progressively decreased by about 80% during the treatment, whereas a secondary rise of FSH levels occurred after the first month of treatment. The patients were orchiectomized after 6 months of treatment. No differences were found between the pre- and postsurgical levels of serum T or in comparison with those of six patients orchiectomized as the first therapeutic measure. Testicular endogenous T concentrations, LH and FSH receptors, and in vitro T production were measured in three testis samples and compared with those values in testis tissue obtained from five control patients. The endogenous levels and in vitro production of T were depressed by over 95% in testes from agonist-treated patients. The small residual T production responded to hCG stimulation as in control patients. Interestingly, no change was found in testicular LH receptor content, but FSH receptors decreased by 80%. The elevation in serum PAP at the beginning of the agonist treatment and the small residual testicular T production after 6 months may not be clinically important. However, they indicate the necessity of comparative long term studies between orchiectomy and GnRH agonists in the treatment of patients with prostatic cancer.