Treatment with an LHRH agonist (HOE-766) alone causes an almost complete blockage of testicular testosterone formation in rat and man. In order to neutralize androgens of adrenal origin, a pure antiandrogen (RU-23908) was given in combination with the LHRH agonist in the rat. At doses where each drug has no or minimal effect alone, prostate and seminal vesicle weight were reduced to 9 and 15% of control after 5 months of combined treatment, respectively. Among the species studied, man is the most sensitive to the inhibitory effect of treatment with LHRH agonists on testicular steroidogenesis. Near castration levels of serum testosterone and 5 alpha-dihydrotestosterone are obtained within 1-2 weeks of daily subcutaneous administration of the LHRH agonist [D-Ser(tbu)6, des-Gly-NH2(10)]LHRH ethylamide (HOE-766) in adult men with cancer of prostate. The decrease in serum androgen levels is accompanied by objective remission of the cancer in approximately 75% of cases. In a preliminary study where the LHRH agonist was administered in combination with the pure antiandrogen RU-23908, it was shown that the antiandrogen does not interfere with the LHRH-induced inhibition of serum androgen levels. The ease of application of this new form of hormonal therapy should permit its use at early stages of the disease and thus reduce the development of metastases and androgen-resistant cell clones.