This study examined the effect of electrical stimulation of the ventral mesencephalic tegmentum versus the dorsal tegmentum region on blood LH levels in freely moving, ovariectomized (OVX) rats, as well as in OVX, estrogen-primed rats. In OVX rats, electrical stimulation (with parameters of 30 Hz, 0.4-msec biphasic pulses, 50–100 µA and 10 s on/off for 1 h) of the ventral tegmentum region with the aim of activating the ventral noradrenergic tract (VNT) failed to affect the pulsatile pattern of blood LH levels characteristic of OVX animals. In sharp contrast, electrical stimulation of the dorsal tegmentum region inside the ascending dorsal noradrenergic tract (DNT) markedly inhibited pulsatile LH release in OVX rats; the mean blood LH levels during the 1-hour stimulation period were significantly decreased when compared with prestimulation control values. Electrical stimulation in the same region near but outside the DNT was ineffective. In OVX rats primed with estradiol benzoate, electrical stimulation in the ventral or dorsal tegmentum region failed to alter the low, nonpulsatile blood levels of LH. The results in OVX rats suggest that selective activation of the ascending noradrenergic fibres of the DNT but not the VNT can inhibit pulsatile LH release in the awake, freely moving animal. These data further support the possible existence of an inhibitory noradrenergic system in the modulation of LH relea