The effect of electrical stimulation of the arcuate nucleus of the hypothalamus (ARH) on the blood levels of luteinizing hormone (LH) was studied in ovariectomized (ovx) rats. Unanesthetized, unrestrained rats were bled continuously (30 or 50 mul whole blood/4-6 min) through indwelling right atrial cannulae for 1 1/2-2h prior to the onset of stimulation. The bleeding was continued for the following three hours, during which electrical stimulation was applied for two 60 min periods separated by a 60 min non-stimulation period. Whole blood was analyzed for LH by radioimmunoassay. Electrical stimulation of the ARH in ovx rats inhibited the episodic LH release characteristic of these animals. The inhibition was rapid in onset, beginning within minutes, and lasted for most if not all of the stimulation period or longer. The blood LH levels returned to control values prior to the second stimulation period during which the inhibitory response was again seen. Stimulation in other hypothalamic areas outside the ARH was generally ineffective, although repeatable inhibition of episodic LH release did occur in 2 of 3 rats stimulated in the periventricular nucleus. In a second study, ovx rats were primed with 5 mug estradiol benzoate/100 g body weight/day for 2 days prior to stimulation. In these rats, stimulation of the ARH or median eminence produced increases in LH release, whereas stimulation in other hypothalamic areas outside the ARH was ineffective. The parameters of electrical stimulation that produced increases in LH release were the same as those that caused the inhibition of episodic LH release. The results demonstrate that electrical stimulation of the ARH inhibits LH release in ovx rats, but increases LH release if the animals have been primed with estrogen.