Two narcotic antagonists, naloxone and naltrexone, significantly elevated serum LH levels in male rats within minutes after their sc injection. The peak increase in serum LH occurred 20 min after the injection. Naloxone increased LH levels up to a dose of 1 mg/kg, after which no further increases were found. A dose of 0.35 mg/kg produced a half-maximal response. The exogenous opioid morphine blocked the increase in LH produced by naloxone in a dose-dependent fashion, suggesting that the specific receptor-blocking effects of the antagonist could account for its enhancement of serum LH levels. The locus of action of naloxone within the hypothalamic-pituitary-LH axis appeared to be at the level of the hypothalamus since the drug had no effect on LHRH-stimulated release of LH by the anterior pituitary and did not block dihydrotestosterone’s suppression of pituitary LH release in vitro. Naloxone also prevented testosterone’s negative feedback inhibition of serum LH in the castrated male rat. The results of these studies suggest that endogenous opioids exist in brain tissue which normally inhibit activity in the hypothalamic-pituitary-LH axis and participate in the androgen-dependent feedback control of LH elaboration by this axis.