To investigate the channels and neurotransmitters in the ventrolateral medulla (VLM) oblongata that are responsible for the maintenance of sympathetic tone and cardio-respiratory reflex regulation. Microinjections of excitant amino acid (glutamate, 100 nl, 100 mM), calcium channel blockers, agonists and antagonists were made throughout the VLM in anaesthetized rats. Arterial blood pressure, sympathetic nerve activity and phrenic nerve discharge was recorded. Responses to activation of the baroreceptor, somatosympathetic, and chemoreceptor reflexes were recorded. Injection sites were marked with albumin-colloidal gold or methylene blue. At the end of recording, brains were removed, fixed, sectioned and stained for gold spots, and counterstained to localize injection sites. Bilateral microinjection of calcium channel antagonists into the rostral VLM (RVLM) had little effect on arterial blood pressure with the exception of Ni++ (an agent that blocks low voltage T-type calcium channels). Ni++ caused a profound fall in arterial blood pressure and sympathetic nerve activity, but did not affect reflex function. By contrast the high voltage channel (P, Q, L, N) blockers increased blood pressure and sympathetic activity following injection into the caudal VLM (CVLM). The agents applied were 8-OHDPAT; 5HT1a agonist, DAMGO; mu opioid agonist, DPDPE; delta opioid agonist and sar9-substance P; substance P agonist. In addition, the effect of changing the inhaled concentration of carbon dioxide was tested. In all cases except injection DAMGO, there was an attenuation of the somato-sympathetic reflex. DAMGO attenuated the baroreceptor reflex. None of these agents affected the chemoreceptor reflex. The data suggest that tonic maintenance of sympathetic activity is mediated, at least in part, by a Ni++ sensitive conductance in RVLM and that the neurochemical heterogeneity found within the inputs to sympathoexcitatory and inhibitory neurons in the RVLM has functional significance, with different neurotransmitters affecting cardiorespiratory reflexes differentially. Further functional studies are required to determine the precise connectivity between different neuronal populations that are subserving different functions in the RVLM.