To study the expression of the carboxy-ter-minal PSD-95/DLG/ZO-1 ligand of nNOS (CAPON) and Dexrasl 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 Dexrasl mRNA level in the spinal cord. The spatial expression of CAPON and Dexrasl mRNA was examined by a com-bination of in situ hybridization (ISH) and immunofluor-escence. 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 dor-sal ho when mature. It then decreased gradually until postnatal week 12, when it presented in the ventral ho. At embryo day 14, Dexrasl mRNA was expressed at low levels, increased during embryo day 16 to day 18 and peaked at postnatal day 1. Spatiotemporal expression of Dexrasl 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 ho at postnatal day 1 but had different subcellular localizations. Co-express-ion of CAPON and Dexrasl 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 synap-togenesis by regulating nNOS to activate Dexrasl.