The effect of naloxone on the negative feedback action of gonadal steroids on luteinizing hormone (LH) release was studied in castrated male and female rats. The reduction by estradiol benzoate or testosterone propionate of elevated serum LH levels in rats ovariectomized for 4 weeks was reversed by a single injection of naloxone. Injection of estradiol benzoate, together with progesterone, similarly reduced serum LH levels in ovariectomized rats, and this inhibition was partially reversed by naloxone. A single injection of testosterone propionate decreased serum LH levels in male rats castrated 48 h earlier, and naloxone completely blocked the androgen-induced inhibition of LH secretion. Chronic administration of either testosterone propionate or morphine sulfate to castrated male rats prevented the postcastration rise of serum LH. The decrease of hypothalamic LHRH in castrated rats was completely blocked by testosterone propionate or morphine sulfate administration. These results are believed to indicate that hypothalamic opiates are involved in gonadal steroid feedback inhibition of LH release.