The neural circuitry of the dorsal horn of the spinal cord forms the basis for the mechanisms of pain and neuralgia. Our lab has made significant inroads in understanding the neuronal connectivity which subserves these sensory phenomena through experiments involving multiple markers to identify interactions between neural elements. In situ hybridization histochemistry was used to examine changes in opioid gene expression in a model of peripheral inflammation and hyperalgesia. No detectable change in preproenkephalin mRNA was visible in spinal dorsal horn neurons at the same time that preprodynorphin mRNA exhibited a dramatic increase. These observations suggest that the two opioid systems are responding differentially to prolonged nociceptive inputs. Afferent input to spinal dynorphin neurons which exhibit an upregulation during peripheral inflammation and hyperalgesia were examined using double label immunocytochemical methodology. Calcitonin gene-related peptide containing primary afferent axons were observed to contact dynorphin neurons in laminae I, II and V. These contacts may provide a direct link between peripheral nociceptive stimuli and opioid neurons in the dorsal horn. The rostral projection of small diameter primary afferents was examined in the lumbar spinal cord. Immunocytochemical staining for calcitonin gene-related peptide demonstrated that labeled afferents in Lissauer's tract and laminae I, II and V traveled at least five segments from their level of entry into the spinal cord. Within the spinal cord, these afferents are unmyelinated. These observations provide new evidence on the extent of small diameter primary afferent projections within the spinal cord.