The central gray region (lamina X) of the lumbar spinal cord in cat was examined by electron microscopy. This region consisted of three morphological zones. Medially, the first zone was comprised of ependyma which surrounded the central canal. The ependyma in the cat spinal cord was similar to most vertebrate spinal ependyma. Secondly, a subependymal zone consisted of glial processes arranged parallel to the long axis of the spinal cord. This glial zone was widest lateral to the central canal and extended approximately 75 μm. The lateral edge of the glial zone intermingled with a neuropil zone, the third zone. The components of the neuropil zone consisted of dendrites, myelinated and unmyelinated axons, synaptic terminals, astrocytes and neurons. The dendrites and neurons generally were oriented parallel with the long axis of the spinal cord. Three synaptic terminal types were categorized according to vesicular morphology, i.e. small round vesicles, flattened vesicles and dense core vesicles. The central gray region has been implicated in nociception and has been shown to receive both primary afferent and supraspinal input. The results from this study are consistent with the central gray region being an area of multiple synaptic inputs which may form the morphological basis of nociceptive processing that ascends to brainstem nuclei.