Glutamate is an amino acid neurotransmitter capable of producing widespread receptor-mediated neuronal excitation. In this experiment, we examined the effect of saline, monosodium glutamate (MSG), phenylalanine and MSG-phenylalanine treatment on TH immunoreactivity in area postrema (AP) of medulla oblangata. An immunocytochemical method was used to visualize catecholaminergic neurons in the AP. Damage of TH neurons in the AP of adult Sprague-Dawley rats was induced by injection of MSG (4 mg/g bw) and was decreased by administration of MSG following phenylalanine treatment (15 mg/g bw). We conclude that phenylalanine protect from the neuroexcitotoxic effect of systemic glutamate.