Carbon monoxide (CO) has been identified as an endogenous biological messenger in the brain. Heme oxygenase catalyzes the metabolism of heme to biliverdin and CO. Recent studies have demonstrated that CO is involved in central cardiovascular regulation and modulates the baroreflex in the nucleus tractus solitarii of rats. The purpose of the present study was to investigate the possible interaction of CO and excitatory amino acids in the nucleus tractus solitarii. In anesthetized male Sprague-Dawley rats, unilateral intranucleus tractus solitarii microinjection of hematin, a heme molecule cleaved by heme oxygenase to yield CO, or excitatory amino acids L-glutamate produced depressor and bradycardiac effects. Similar cardiovascular effects were observed with several agonists for ionotropic glutamate receptors such as N-methyl-D-aspartate (NMDA), (+/-)-alpha-amino-3-hydroxyl-5-methylisoxazole-4-propanoic acid (AMPA), kainic acid and for metabotropic glutamate (mGlu) receptors, trans-(+/-)-1-amino-(1S,3R)-cyclopentanedicarboxylic acid (ACPD). Among these agonists, prior administration of the heme oxygenase inhibitor, zinc deuteroporphyrin 2,4-bis glycol (ZnDPBG) (1 nmol), significantly attenuated the cardiovascular effects of hematin, L-glutamate and ACPD. Furthermore, the cardiovascular effects of ACPD were prevented by the selective mGlu receptors antagonist L-2-amino-3-phosphonoprionate (L-AP3). However, pretreatment with ZnDPBG failed to prevent the cardiovascular responses to microinjection of NMDA, AMPA and kainic acid. On the other hand, prior administration of the NMDA receptor antagonist, diazocilpine (MK-801), or (+/-)-2-amino-5-phosphonopentanoic acid (APV) attenuated the depressor and bradycardiac effect of hematin. These results demonstrated that mGlu receptors may couple to the activation of heme oxygenase via the liberation of CO to participate in central cardiovascular regulation. They also suggested that CO and excitatory amino acids may interact in the nucleus tractus solitarii of rats.