Sympathetic nervous outflow arises from sympathetic preganglionic neurones (SPNs) in the central autonomic area (CAA) and intermediolateral cell column (IML) of the thoracic and upper lumbar spinal cord. Descending serotoninergic axons heavily innervate the CAA and also the IML, where they form synapses with SPNs (Bacon & Smith, 1988). Functionally, serotonin directly excites SPNs and also induces indirect IPSPs in SPNs in spinal cord slices (Lewis et al. 1993), suggesting excitation of local inhibitory interneurones. We have identified interneurones within the vicinity of the IML (Deuchars et al. 2001) and within the CAA (Deuchars et al. 2005) that are likely to innervate SPNs. Interneurones in the CAA inhibit SPNs via a GABAergic connection. In this study we are investigating if serotonin containing terminals innervate GABAergic neurones in the IML and CAA. Targets of serotonin in thoracic spinal cord were investigated using immunohistochemistry on transgenic reporter mice expressing GFP in cells containing glutamic acid decarboxylase (GAD)65 (De Marchis et al. 2004). Adult mice (n=5) were anaesthetised with an intraperitoneal injection of Sagatal (60mg/kg) and perfused transcardially with 4% paraformaldehyde in 0.1M phosphate buffer, pH 7.4. Thoracic spinal cord was sectioned at 50µm on a vibratome (Leica). Serotoninergic terminals were localised using rabbit anti-5-HT (1:500, Neuromics Inc, USA) and visualised with donkey anti-rabbit Alexa⁵⁵⁵ (1:1000, Invitrogen, UK). Stained sections were analysed for close appositions between serotoninergic terminals and GFP expressing cells using an epifluorescence microscope. The distribution of GFP expressing cells was similar to that observed in our previous studies of cells expressing GAD detected by in situ hybridisation (Deuchars et al. 2005). Numerous GFP containing cells were visible in the CAA (dorsal and lateral to the central canal). Although labelled cells were sparse within the IML, they were more common around the edges, similar to the location of pre-sympathetic interneurones. Serotonin containing terminals were highly concentrated in the IML and CAA as previously described by Bacon & Smith (1988) and were visible at low magnification in the vicinity of GFP expressing cells. High power examination revealed serotoninergic terminals were indeed closely apposed to both GFP cell bodies and dendrites. These results suggest that 5-HT containing terminals can innervate GABAergic neurones in the vicinity of the IML and in the CAA. This provides an anatomical substrate for the IPSPs induced in SPNs in spinal cord slices by 5-HT. Descending pathways may therefore exert control of sympathetic outflow indirectly via local interneurones as well as through direct actions on SPNs.