The effects of monosodium L-glutamate (MSG) on the brain of mice were examined by light-microscopy under the intended conditions of use of this material. Basal diet containing 5, 10 or 15% w/w MSG was fed to pregnant, lactating and weanling mice ad libitum, resulting in the ingestion of 14.0, 42.8 or 42.0 g/kg body wt. of MSG a day respectively. Mice at the same developmental stages were given the basal diet and 5% w/w MST in aqueous solution ad libitum, the maximum ingestion of MSG being 16.9, 31.4 and 26.8 g/kg body wt. a day respectively. Light-microscopy examination revealed no pathological changes in the hypothalamic arcuate nuclei (AN) of all treated mice, i.e., pregnant females and their fetuses, lactating females and their sucklings, and weanlin mice. Pregnant females and their fetuses, and weanling mice, injected subcutaneously with either 5.0 or 4.0 g/kg body wt. of MSG, showed "Glu-type lesions" in the hypothalamus.