By combining two parallel sets of studies, we present evidence on the functional assembly of neuronal circuitry underlying the processing of nociceptive sensations in the dorsal horn of the spinal cord. One set of observations was made using a transgenic mouse line expressing a membrane-targeted version of green fluorescent protein (GFP) selectively in epidermal primary afferents. The second set were made using whole cell patch recordings from spinal cord slices isolated from juvenile rats. All animals used were deeply anaethestised (pentobarbitone 80mg/kg, for adult mice or ether for juvenile rats) prior to either vascular perfusion (adult mice) for histology or decapitation (juvenile rats) and spinal cord slice preparation for whole-cell patch studies. Standard immunochemical and intracellular labelling methods were employed. In transgenic mice in which the Thy1.2 promoter was used to drive expression of GFP, exclusively small diameter dorsal root ganglion (DRG) neurones expressed GFP and these also bound IB4 lectin. GFP expression was detected in their peripheral axons selectively terminating in the epidermis whereas their central terminals were restricted to LII. No colocalisation with substance P was found. Many IB4 binding primary afferents terminate in glomerular synapses forming contacts with spines on stalked cells. Whole cell patch recordings from LII neurones (n=55) in rat lumbar spinal cord slices revealed that many had the morphology of stalked cells (n=10) and were excited by capsaicin. This excitation persisted in the presence of TTX and could be blocked by AMPA receptor antagonists (NBQX, 10μM). In the absence of magnesium the post-synaptic responses were prolonged and this could be reversed by an NMDA antagonist. Intracellularly labelled stalked cells were shown to express the glutamate vesicular transporter and hence, are excitatory interneurones. These observations provide strong evidence for the existence of a system in which epidermal primary afferents are activated by noxious heat, and in which centrally projecting afferents synapse on LII excitatory interneurones. A parallel system involving substance P containing primary afferents innervating LI spinothalamic neurones is also nociceptive. We hypothesize that epidermal neuronal circuitry may be involved in detection of superficial burning sensations such as would occur in sun-burn. In contrast, substance P containing circuitry is more concerned with tissue injury and inflammation. These findings will have important implications for the treatment of pain.