The aim of this study was to investigate further the possible role of nerve cells of the arcuate nucleus in the regulation of growth hormone (GH) and prolactin (PRL) secretion. Newborn male rats were treated with monosodium-L-glutamate (MSG) (4 g/kg body weight subcutaneously) or NaCl (0.5 g/kg) on alternate days for the first 10 days of life, and were used 8-9 weeks afterwards. beta-Endorphin-like immunoreactivity in the arcuate nucleus decreased by 54% after MSG treatment. The dopamine content of the median eminence was reduced to 55% of that in NaCl-treated litter mates. No significant change of dopamine content was found in the other hypothalamic regions examined. Basal levels of plasma PRL were unaltered. Intravenous injection of morphine to urethane-anesthetized animals was 3 times more potent in stimulating PRL secretion in MSG-treated rats, while it was 2.5 times less potent in releasing GH. These data suggest that lesioning of the arcuate nucleus by MSG treatment damages neurons inhibiting PRL secretion, as well as neurons facilitating GH secretion.