These experiments were performed to investigate the effect of saline, monosodium L-glutamate(MSG), MSG-phenylalanine, and/or phenylalanine on TH-immunoreactivity in arcuate nucleus of hypothalamus and hind limb area of cerebral cortex in rats using the immunohistochemical methods. The result were as follows: 1. TH-immunoreactive neurons in hind limb area of cerebral cortex and arcuate nucleus of hypothalamus were decreased in MSG treated group compared to the saline treated group and also lesions in arcuate nucleus and median eminence of hypothalamus were increased with MSG treatment 2. TH-immunoreactive neurons in median eminence and arcuate nucleus were increased in phenylalanine treated group compared to the saline treated group and also neurons were more increased in lamina V of hind limb area of cerebral cortex with phenylalanine treatment. 3. TH-immunoreactive neurons in median eminence and arcuate nucleus were decreased in MSG-phenylalanine treated group compared to the phenylalanine treated group and increased compared to the MSG treated group. In lamina V of hind limb area of cerebral cortex, TH-immunoreactive neurons were more decreased in MSG-phenylalanine treated group than that of the phenylalanine treated group, and more increased than that of MSG treated group. These experiments indicated that TH-immunoreactive neurons in hypothalamus and cerebral cortex were increased due to the activation of phenylalanine and decreased by suppressing activation of phenylalanine through MSG treatment.