Neonatal administration of monosodium glutamate (MSG) produces necrosis of circumventricular structures, including perikarya in the medial-basal hypothalamus that contain beta-endorphin (BEND) and met-enkephalin. Since neonatal MSG treatment alters morphine analgesia, the present study examined neonatal MSG effects upon opioid analgesia observed following either BEND or d-ala d-leu enkephalin (DADL). Rats treated with either MSG or vehicle over the first ten post-natal days, were surgically prepared with a lateral ventricle cannula at 100 days of age. Respective groups received central injections of either BEND (0, 0.1, 0.5 or 1.0 microgram) or DADL (0, 4, 20 or 40 micrograms), and jump thresholds were assessed 15, 30, 45 and 60 min thereafter. Following testing, selected MSG-treated and control animals were prepared for BEND immunocytochemistry. While the magnitude, duration and sensitivity of BEND analgesia on the jump test failed to differ between groups, MSG-treated rats displayed a 10-fold leftward shift in sensitivity and a 200-300% increase in the magnitude of DADL analgesia. Immunocytochemical analysis indicated that MSG treatment depleted perikarya in the medial-basal hypothalamus, periventricular thalamic fibers and periaqueductal gray terminal fields that contained BEND. The differential effects of MSG treatment upon opiate and opioid analgesia are discussed in terms of possible alterations in opiate receptor subpopulations.