This study was done to investigate the effects of microinjections of adrenomedullin (ADM), a vasoactive neuropeptide, in the rostral ventrolateral medulla (RVLM) on mean arterial pressure (MAP) and heart rate (HR) in urethane-anesthetized rats, and to assess the potential roles of glutamate and nitric oxide (NO) in these effects. Unilateral injections of ADM (0.01 or 0.1 pmol) into the RVLM significantly increased MAP and HR in a dose-dependent manner, while ADM at 0.001 pmol was ineffective. Microinjections of ADM (0.01 pmol) outside the RVLM had no effects on MAP or HR. Co-injections of a putative ADM receptor antagonist, ADM_22-52 (0.01 pmol), abolished the increases in MAP and HR evoked by ADM (0.01 pmol). The vasopressor effects of ADM (0.01 pmol) in the RVLM were abolished by co-injections of either dizocilpine hydrogen maleate (MK-801, a selective NMDA glutamate receptor antagonist, 500 pmol) or 6-cyano-7-nitroquinoxaline-2, 3-dione (CNQX, a selective non-NMDA glutamate receptor antagonist, 50 pmol). The ADM-induced vasopressor effects were also abolished by co-administration of either 7-nitroindazole sodium salt (7-NiNa, a selective neuronal NO synthase inhibitor, 0.05 pmol) or methylene blue (a soluble guanylyl cyclase [sGC]inhibitor, 100 pmol). These results suggest that ADM in the RVLM stimulates increases in MAP and HR through an ADM receptor-mediated mechanism. These effects are mediated by glutamate via both NMDA and non-NMDA receptors. NO, derived from neuronal NO synthase, also contributes to the ADM-induced vasopressor effects via an sGC-associated signaling pathway.