The rostral ventrolateral medulla (RVLM) regulates sympathetic vasomotor outflow and reflexes. Intracerebroventricular neuromedin U (NMU) increases sympathetic nerve activity (SNA), mean arterial pressure (MAP), and heart rate (HR), but the central nuclei that mediate these effects are unknown. In urethane-anesthetized, vagotomized, and artificially ventilated male Sprague-Dawley rats (n = 36) the effects of bilateral microinjection of NMU (50 nl, each side) into RVLM on cardiorespiratory variables, somatosympathetic reflex, arterial baroreflex, and chemoreflex were investigated. Microinjection of NMU into RVLM elicited a hypertension, tachycardia, and an increase in splanchnic SNA (SSNA) and lumbar SNA (LSNA) at lower doses (25 and 50 pmol). At higher dose (100 pmol), NMU caused a biphasic response, a brief hypertension and sympathoexcitation followed by prolonged hypotension and sympathoinhibition. The peak excitatory and inhibitory response was found at 100 pmol NMU with an increase in MAP, HR, SSNA, and LSNA of 36 mm Hg, 20 beats per minute, 34%, and 89%, respectively, and a decrease of 33 mm Hg, 25 beats per minute, 42%, and 52%, respectively, from baseline. NMU, in the RVLM, also increased phrenic n erve amplitude and the expiratory period and reduced the inspiratory period. NMU (100 pmol) attenuated the somatosympathetic reflex and the sympathoexcitatory and respiratory responses to hypoxia and hypercapnia. After NMU injection in RVLM, the maximum gain of the SSNA baroreflex function curve was increased, but that of the LSNA was reduced. The present study provides functional evidence for a complex differential modulatory activity of NMU on the cardiovascular and reflex responses that are integrated in the RVLM.