Consumption of monosodium glutamate has long been considered to precipitate headaches in susceptible patients. In this study the direct effects of glutamate and its metabolite, glutamine, on arterial contractility were examined using rings of rabbit aorta. In a high concentration glutamate caused significant concentration-dependent contractions (EC 50 , 10 -1 M; maximum tension, 188.4±33.3 mg wt tension/mg tissue). Agonists and antagonists for a-adrenergic, histaminergic, serotonergic, cholinergic, and GABA-nergic receptors as well as inhibition of prostaglandin synthesis failed to influence glutamate contractions. At high concentrations (10 -5 M) the calcium channel blocker, verapamil, inhibited the glutamate response. Glutamate and glutamine both exhibited concentration dependent relaxation of norepinephrine (NE), phenylephrine (PE), histamine, serotonin (5-HT), and prostaglandin F 2a (PGF 2a )-induced contractions. Kainic acid (10 -4 M), an agonist of one subpopulation of central glutamate receptor, potentiated glutamate-induced vasoconstriction; a higher concentration (10 -3 M) produced an irreversible inhibition of glutamate contractility. Only the central glutamate receptor antagonist, ketamine (10 -4 -10 -2 M), induced a reversible, concentration dependent inhibition of glutamate-induced contractions. Glutamate contractility was not dependent on extracellular calcium, an intact endothelium or neuronal function. These results demonstrate a direct effect of glutamate on peripheral arterial tone. Dietary consumption of large quantities of MSG may represent a serious health hazard to certain individuals with pre-existing vascular disease.