Plasma LH was measured by radioimmunoassay inblood withdrawn through atrial cannulas at 10-min intervals in long-term ovariectomized (OVX) rats. A subcutaneous (s.c.) injection of estradiol benzoate or an intravenous (i.v.) injection of 17β-estradiol inhibited the pulsatile discharge of the hormone and depressed the plasma LH concentration. Neither progesterone nor small amounts of rat prolactin altered the pulsatile rhythm or lowered plasma LH levels. A s.c. injection of nicotine tartrate at a dosage which blocks the pro-estrous rise of LH caused a marked depression in plasma LH concentration. In rats which still exhibited pulsatile LH release after being subjected to partial or total deafferentation of the medial basal hypothalamus (MBH) 4–6 weeks prior to ovariectomy, injections of estradiol benzoate, 17β-estradiol and nicotine gave results similar to those seen in non-deafferented OVX animals. 17β-estradiol had a partial inhibitory effect on pituitary LH release in response to an injection of synthetic LHRH. The results suggest that the inhibitory effects of estrogen and nicotine on LH release are exerted via blockade of LHRH discharge from the MBH and that estrogen also inhibits LH release by an action on the adenohypophysis.