To determine whether endogenous opiates exert an inhibitory action on LH secretion during the menstrual cycle, LH increments were examined in response to a specific opiate antagonist, naloxone, infused at 1.6 mg/hr for 4 hrs. Naloxone had no discernible effect on LH release during the early follicular phase. By contrast, a significant LH increment was observed in both the late follicular and mid-luteal phases of the cycle but the patterns of LH increment were distinct. Late follicular subjects exhibited slow, progressive increases in LH, while prompt, episodic and quantitatively greater LH increments were evident in mid-luteal phase subjects. These observations suggest that endogenous opiates are involved in the regulation of LH secretion during the high estrogen and estrogen-progesterone phases of the menstrual cycle.