In a seminal paper “Discharges in mammalian sympathetic nerves”, Adrian et al.(1932) described the rhythmic nature of sympathetic nerve discharges. Since then some of the mechanisms underlying sympathetic rhythm generation have been unravelled, yet many aspects of their functional significance remain elusive. One of the rhythms documented by Adrian et al. was that related to central respiratory drive, which may be relevant to the coordination of cardiovascular and respiratory functions. Work from Polosa’s laboratory (e.g., Priess & Polosa, 1975) documented patterns of respiratory-related activity of single sympathetic preganglionic neurones (SPN). Such activity may arise from the interaction of rhythmic and tonic components at the level of the SPN. This contention is supported by the observation of Gilbey et al. (1986) that quiescent SPNs brought to threshold by the ionophoretic application of glutamate could have firing patterns related to central respiratory drive. Probably the first indication that rhythmic activity might arise within sympathetic networks themselves, rather than from rhythmic inputs to tonic sympathetic-tone generating networks, came from the work of Green & Heffron (1967; see Barman & Gebber, 2000). Subsequently, Gebber became a main proponent of the sympathetic rhythm generator hypothesis and identified sympathetic rhythms within medullary networks (see Barman & Gebber, 2000). However, is now becoming established that some sympathetic rhythm generating substrates are located within the spinal cord. Yoshimura et al. (1987) observed noradrenaline-induced membrane potential oscillations in SPNs recorded in slice preparations. These remained in the presence of tetrodotoxin. Chitz et al. (1997) reported somatic motor coupled postganglionic rhythmic sympathetic discharges in an in situ perfused spinalised rat. Recent work from Sue Deuchars laboratory has demonstrated, in sympathetic regions of rat spinal cord slices, 5-HT-driven rhythmic population activity . Marina et al. (2006) noted that intrathecal application of 5-HT to the thoracolumbar spinal cord in lower thoracic spinalised anaesthetised rats generated a ~ 1 Hz rhythm in sympathetic activity supplying the rat tail circulation. The rhythm was similar to the spontaneous rhythm observed in an intact anaesthetised animal (T-rhythm, see Gilbey, 2007). Entrainment of such spinal rhythms may be a mechanism for the coupling of networks involved in controlling various motor outflows; e.g., sympathetic and respiratory. Indeed studies from my laboratory have shown that whereas in the absence of an entraining input the T rhythm discharges recorded from pairs of single postganglionic neurones innervating the rat tail circulation can be dissociated, they can be entrained by various input(s); e.g., those related to central respiratory drive (see Gilbey, 2007). It can be hypothesised that sympathetic rhythm generation within the spinal cord allows for the flexible coupling of sympathetic activity with other networks; e.g., respiratory and locomotor. The degree of synchronisation of rhythmic sympathetic activity can depend on the strength of the entraining input and its timing with respect to the phase of the rhythm generator. Regulating the degree of synchronisation of sympathetic discharges in this manner may increase the efficacy of transmission along the efferent pathway from spinal cord to effector. Furthermore synchronisation of activity of SPNs may lead to the recruitment of quiescent SPNs (see Gilbey 2007). These hypotheses remain to be tested.