We studied the antifertility effects of a potent gonadotropin-releasing hormone agonist (LHRHA) D-Trp -Pro -N-ethylamide-LHRH in 8 normal men who received daily subcutaneous injections for 6-10 weeks. Plasma testosterone levels fell substantially in all 8. Plasma 17-hydroxyprogesterone and serum estradiol-17beta levels decreased concordantly with plasma testosterone. Impotence developed in 5 men between the 6th and 7th weeks of treatment with resolution in each case within 2 weeks of cessation of treatment. Serum gonadotropin levels also fell during treatment briefly rebounding above basal levels when therapy ended. Sperm density and motility fell to a nadir during the 7th-18th weeks after therapy. In 6 subjects sperm levels fell to 6x10 sperm/milliliter or less and in the other 2 they decreased 70 and 86% respectively below basal mean values. Sperm density returned to pretreatment levels in all men during the 10-14 weeks of recovery. These results are consistent with LHRHA-induced pituitary desensitization but do not exclude a direct inhibitory effect of LHRHA on testicular steroidogenesis and spermatogenesis. (authors)