The effect of chronic treatment with the luteinizing hormone-releasing hormone (LH-RH) agonist Buserelin (Hoechst AG, Frankfurt/Main, West Germany) ([D-Ser(TBU)6,des-Gly-NH2(10)]LH-RH ethylamide) administrered by nasal spray (200 or 500 micrograms, twice daily) or subcutaneously (50 micrograms daily) for periods of 1 to 8 months was studied on serum sex steroids and LH levels in 18 patients with cancer of the prostate. Basal serum testosterone concentration decreases to 71.1 +/- 18.3 (NS) and 28.6 +/- 9.3%, (P less than 0.01) of control in patients receiving the 200-micrograms and 500-micrograms dose by nasal spray, respectively. In patients treated subcutaneously, a more rapid inhibition of serum testosterone levels to 19.6 +/- 6.4% of control (P less than 0.01) is observed. The finding of decreased levels of 17-OH-progesterone, testosterone, and dihydrotestosterone in the presence of unchanged pregnenolone concentration indicates that the decrease in androgen biosynthesis induced by Buserelin treatment is due to a blockage at the level of 17-hydroxylase and 17,20-desmolase activities. The present data indicate that chronic administration of Buserelin could be a safe and effective means of reducing serum androgens in patients with cancer of the prostate.