The hormonal effects of a delayed-release intramuscular (IM) preparation of D-Trp-6-LH-RH (TRP-6 microcapsules) were investigated in men aged 65-75 years. Six men with a prostatic adenoma (group A) received subcutaneously (SC) 500 micrograms TRP-6/day from day 0 to day 6. Seven men with a prostatic carcinoma (group B) were given the same SC treatment, but in addition received on days 7, 28 and 56 an intramuscular injection of 3 mg TRP-6 microcapsules. During SC treatment, plasma TRP-6 levels rose to 2-4 ng/ml. Plasma LH peaked on day 1, whereas high testosterone levels were maintained from day 2 to day 4, and then fell abruptly on day 6. In five subjects from group B, plasma TRP-6 was detectable in all plasma samples from day 10 to day 49. Testosterone levels were in the castrate range after day 24. Mean LH level decreased progressively from day 14 to day 56. These data provide evidence for the efficacy of periodic administration of TRP-6 microcapsules for suppressing testicular secretion.