To study the expression of the carboxy-terminal PSD-95/DLG/ZO-1 ligand of nNOS (CAPON) and Dexras1 mRNA during development in the spinal cord of rats, real-time polymerase chain reaction (Real-time PCR), as a quantitative method, was used to study the developmental expression of CAPON and Dexras1 mRNA level in the spinal cord. The spatial expression of CAPON and Dexras1 mRNA was examined by a combination of hybridization (ISH) and immunofluorescence. During the development of the spinal cord, CAPON mRNA was expressed in low levels from embryo day 14 to day 18. At postnatal day 1, it reached the peak and was expressed in the part which will become the dorsal horn when mature. It then decreased gradually until postnatal week 12, when it presented in the ventral horn. At embryo day 14, Dexras1 mRNA was expressed at low levels, increased during embryo day 16 to day 18 and peaked at postnatal day 1. Spatiotemporal expression of Dexras1 mRNA was similar to CAPON as confirmed by correlation analysis and colocalization. CAPON and neuronal nitric oxide synthase (nNOS) was expressed within the same cells of the dorsal horn at postnatal day 1 but had different subcellular localizations. Co-expression of CAPON and Dexras1 mRNA in myeloid tissue during development process of rat indicates that the adaptor protein, CAPON may play a probable role in differentiation of neurons, synaptic plasticity and synaptogenesis by regulating nNOS to activate Dexras1.