The direct effect of gonadotropin-releasing hormone (GnRH) and its agonist on testicular LH receptor and steroidogenesis was studied in hypophysectomized immature and adult rats. Hypophysectomized rats were treated daily with varying doses of GnRH or [des-Gly10,D-Leu6(N alpha Me)Leu7, Pro9-NHEt]GnRH(a potent agonist). Some animals were also treated concomitantly with FSH, PRL, GH and/or LH to prevent the hypophysectomy-induced loss of testicular LH receptor and steroidogenic capacity. At the end of 5 days of treatment, testicular LH/hCG receptor concentration was measured by a [125I]-hCG-binding assay and steroidogenic responsiveness was determinded by in vitro incubations. GnRH and the GnRH agonist reduced testicular LH receptor in control and FSH-treated hypophysectomized immature rats. As little as 0.5 microgram agonist/day induced a greater than 40% decrease in the LH receptor content, whereas GnRH was less potent, with 50 micrograms/day inducing about a 50% decrease. The inhibitory effect of GnRH was shown to be the result of decreases in the concentration of LH receptor rather than changes in the receptor affinity (Kd = 1.1 X 10(-10)M). GnRH did not interfere with the [125I]hCG receptor assay. Treatment with PRL, GH, and FSH, alone or in various combinations, increased the testicular LH receptor content. The stimulatory effect of these pituitary hormones was depressed by concomitant treatment with the GnRH agonist. Similar inhibitory effects of GnRH and the agonist on testicular LH receptor were demonstrated in adult hypophysectomized rats. In vitro studies demonstrated that treatment with the GnRH agonist in vivo inhibited both basal and hCG-stimulated androgen production in FSH-primed immature hypophysectomized rats. Associated with decreases in androgens (testosterone and androstenedione) and reduced androgens (dihydrotestosterone, androstanediol, and androsterone), there was marked suppression of 17 alpha-hydroxylated precursors and C-21 steroid intermediates in animals treated with the GnRH agonist, thus suggesting that the inhibitory effect of the GnRH agonist was associated with possible defects in 17 alpha-hydroxylase and side-chain cleavage enzymes. Likewise, treatment with the GnRH agonist inhibited in vitro testicular steroidogenic responses in adult hypopysectomized rats. These results demonstrate the extrapituitary inhibitory effect of GnRH on testicular LH receptor content and Leydig cell steroidogenesis in immature and adult hypophysectomized rats.