Our understanding of the role that spinal cord interneurones play in influencing and integrating outflow from the spinal cord is somewhat limited, especially when a more global role involving more than one output is considered. We have identified a novel group of interneurones in the central autonomic area (CAA) of the spinal cord that directly innervates sympathetic preganglionic neurones (SPNs) to exert an inhibitory influence (Deuchars et al., 2005). Using electrophysiology combined with dye filling of recorded neurons and immunohistochemistry, we have determined the passive electrophysiological and action potential firing properties of CAA interneurones and then analysed their axonal projection patterns and innervation targets. Wistar rats (10-14 days old) were terminally anaesthetised with urethane (2g/kg, i.p.) and transcardially perfused with ice cold 215 mM sucrose aCSF (see Deuchars et al. 2005 for methods). Whole cell patch clamp recordings were made in current clamp mode from 132 CAA interneurones in thoracic spinal cord sections. Neurons were characterised then filled with rhodamine and neurobiotin and recovered/reconstructed for light and electron microscopy. This was combined with immuno- gold labelling for choline acetyltransferase, which labels all SPNs and motoneurones. The majority of CAA neurones (101/132) had an Ih, 12/132 had an Ia, 14/132 expressed both and 5/132 expressed neither. The action potential duration and/or AHP amplitude of those expressing an Ih was significantly different (p<0.05) from the other neurones in this region. Single CAA interneurones had extensive axonal arborisations within the intercalated nucleus and intermediolateral cell column (the sites of the majority of SPNs and their dendrites) and within the ventral horn, the location of motoneurones. Using correlated light and electron microscopy, we identified many close appositions of interneuronal axon with ChAT labelled structures. To confirm that these close appositions were direct synaptic contacts onto ChAT-labelled SPNs and motoneurones, tissue was taken to the electron microscopic level using landmarks in the sections to correlate light and electron micrographic structures. At the electron microscopic level, these close appositions were confirmed as labelled interneuronal synaptic terminals forming synapses with ChAT-immunoreactive dendrites or, on some occasions, cell somata in both autonomic regions and onto motoneurones. This complexity in the innervation patterns of CAA interneurones supports the possibility that they may be involved in integration of autonomic and somatic motor outflow from the spinal cord.