Vagal preganglionic neurones innervating the heart are located within the dorsal vagal nucleus (DVN) and the nucleus ambiguus (NA) but the relative importance of these two sites varies between species with increasing importance of the nA compared to the DVN in the move from fish to mammal. In mammals, preganglionic neurones in the nA have small myelinated axons and are responsible for the major changes in heart rate whilst those in the DVN have non-myelinated axons and mediate some chronotropic, dromotropic and inotropic effects. Numerous studies over the last decade have demonstrated that serotonin (5-HT) has important influences at multiple sites responsible for controlling autonomic outflows. These include the nucleus tractus solitarius (NTS) the site of termination of cardiorespiratory sensory afferent fibres, the cardiac preganglionic neurones within the nA and DVN and within the rostral ventrolateral medulla (RVLM) where sympathetic premotor neurones are located. A series of studies in our Lab have demonstrated the roles of some of the numerous 5-HT receptor subtypes in these brainstem regions involved in control of the heart. Intracisternal application of selective ligands has been used to study the effect of 5-HT receptor subtypes on heart rate and its reflex control, whilst electrophysiological studies have delineated their location and cellular mechanisms of action. Activation of 5-HT_1A receptors potentiated the bradycardias evoked by aortic baroreceptors, cardiopulmonary and upper airway receptors but not arterial chemoreceptors, whereas 5-HT_1B/D receptors had opposing actions. Within the DVN nA and NTS activation of 5-HT_1A receptors could excite or inhibit neurones but, surprisingly, only the excitations were antagonised by 5-HT_1A receptor antagonists (Wang et al, 1997). 5-HT₂ receptors also have differential effects and this relates to the class of NTS neurone, excitatory effects are mediated by 5-HT_2B receptors whilst inhibitory effects are mediated by 5-HT_2C receptors (Sévoz-Couche et al., 2000). Blockade of brainstem 5-HT₃ receptors attenuates the reflex bradycardia evoked by upper airway and cardiopulmonary afferent simulation. This is compatible with electrophysiological data at the level of both the DVN and NTS where activation of 5-HT₃ receptors excites neuronal activity by facilitating release of glutamate from a presynaptic site (Wang et al, 1998). Whether the glutamate is of neuronal or possibly glial origin remains to be determined. However, the latter is a possibility since vagal- and baro-sensitive NTS neurones have been shown to receive few direct synaptic contacts from 5-HT containing boutons (Llewellyn-Smith et al., 2004). Most recently, blockade of 5-HT₇ receptors has been demonstrated to markedly attenuate the falls in heart rate evoked by stimulating cardiopulmonary receptors, arterial baroreceptors and chemoreceptors (Kellett et al., 2004). Thus, 5-HT plays a critical role in the control of vagal outflow to the heart, however, why so many different receptors seem to be involved, and their relative functional roles remains to be resolved.