Neonatal administration of monosodium glutamate (MSG) produces neurotoxic degeneration of the retina and medial-basal hypothalamus, including the arcuate nucleus. Since this hypothalamic area contains the only neuronal cell bodies in brain which contain adrenocorticotrophic hormone (ACTH) and β-lipotropin (β-LPH) and β-endorphin, destruction of these cells by MSG may interfere with pain responses mediated by nerve fibers arising from these perikarya. The present study examined whether MSG-treated rats, as compared to Iittermate controls, exhibited concomitant changes in the immunocytochemical distribution of ACTH and β-LPH, and their reactivity to several analgesia-inducing manipulations. Although MSG-treated rats did not differ from control rats in their baseline reactivity to electric shock, they displayed an inability to exhibit analgesia following acute exposure to cold-water swim stress. In addition, MSG-treated rats showed an attenuated analgesic response following morphine administration. However, the analgesia elicited by either abrupt food deprivation, or the glucoprivic stress of 2-deoxy-Z)-glucose, was unaffected by neonatal MSG treatment. Concomitant with these selective analgesic deficits, MSG-treated rats displayed a marked immunocytochemical reduction in ACTH/β-LPH perikarya and terminals in brain, but not pituitary. These data indicate that multiple pain-inhibitory systems exist, and that some rely upon an intact medial-basal hypothalamus to produce analg