Recently, it has been demonstrated that application of capsaicin to the spinal cord produces EPSPs in some neurones, and that this effect is lost in animals subjected to a prior dorsal rhizotomy (Baccei et al. 2003). Hence, it is argued that application of capsaicin to spinal cord slices can be used to identify neurones that integrate responses to noxious thermal stimuli. The aim of the present study was to determine whether (A) these neurones form a morphologically homogeneous group; and (B) are inhibited by a µ-opioid agonist.Juvenile rats were anesthetised and decapitated, and parasagital slices of the lumbar spinal cord were prepared with attached dorsal roots, dorsal root ganglia and peripheral nerves. Intracellular recordings were made from neurones in LI-LIII. Their responses to bath application of capsaicin (1 and 10mM) were tested. The reponses of some to the µ-opioid agonist DAMGO (1mM), were also tested. Some neurones were labelled by intracellular injection of Neurobiotin for subsequent morphological analysis.Of 46 neurones tested, 13 produced EPSPs to application of capsaicin. These responses rapidly desensitised to repeated capsaicin application. Six of these neurones were also tested with DAMGO and 3 were directly hyperpolarised. Six neurones labelled by intracellular injection that producing EPSPs to capsacin had variable morphology and were distributed throughout LI-III. Two, which were also hyperpolarised by DAMGO, had the morphology of stalked cells with soma at the LILII boarder and dendrites that were mainly ventrally directed and covered in spines.VR1 receptors have been shown on both substance P and IB4 lectin binding dorsal root ganglion neurones (Guo et al.1999). The morphology of spinal neurones processing input from afferents signalling noxious heat, and the effects of µopioids on these circuit elements, suggests that there is a complex parallel processing of nociceptive thermal information in the outer dorsal horn.