T he role of ascending noradrenergic projections of medullary origin in regulating the activity of tuberoinfundibular neurons of the hypothalamic paraventricular nucleus (PVN) was examined in pentobarbital-anesthetized male Sprague-Dawley rats. Discrete electrical stimulation of either the Al or the A2 noradrenaline cell group areas of the caudal medulla enhanced the probability of firing in a substantial proportion of antidromically identified tuberoinfundibular PVN cells tested. Notably, no inhibitory effects were observed. Destruction of the PVN noradrenergic terminal plexus by local application of the neurotoxin 6-hydroxydopamine 1 day prior to electrophysiological experiments abolished the effects of both Al and A2 stimulation. These findings indicate that noradrenergic afferents can exert a facilitatory influence on the activity of a population of tuberoinfundibular PVN neurons, thus supporting earlier suggestions that central noradrenergic structures can enhance the release of certain anterior pituitary hormones.