The brains of 9 infant rhesus monkeys were examined for acute cytopathology following treatment with monosodium L-glutamate (MSG) (6 infants) or NaCl (3 infants). Hypothalamie lesions identical to those described elsewhere in MSG-treated rodents were identified by light microscopy and verified with the electron microscope in each infant monkey given MSG. No cellular pathology was detected in the hypothalami of NaCl-treated controls. Infants given relatively low oral doses of MSG (1 and 2 g/kg) sustained small focal lesions confined primarily to the rostro-ventral aspect of the infundibular nucleus. Those treated with high subcutaneous doses developed lesions which spread throughout and sometimes beyond the infundibular nucleus. At all doses tested (1–4 g/kg) and by either route of administration, rapid necrosis of neurons (within 5 hours) was a striking feature of the MSG-induced reaction pattern. Blood glutamate curves from time of treatment to time of sacrifice (5 hours) suggest that the threshold for lesion formation in 1-week-old rhesus monkeys may be in the range of 20 mg%. Differences in research approach which might account for the failure of others to locate the lesions we describe are discussed as are some of the toxicological implications of our findings.