The distribution of five N-methyl-D-aspartate (NMDA) receptor channel subunit mRNAs in the mouse spinal cord from embryonic day 13 (E13) through postnatal day 56 (P56) was semiquantitatively examined at the cervical level via in situ hybridization with subunit-specific oligonucleotide probes. Signals for the epsilon 1 subunit mRNA were restricted to the most ventral portion of the spinal cord during embryonic stages. They extended to all laminae of the spinal cord except for the lamina 2 (substantia gelatinosa) during postnatal development. A wide expression of the epsilon 2 subunit mRNA was found in the spinal gray matter from E13 through neonatal stages, but the signals became restricted to the lamina 2 by P21. No significant signals for the epsilon 3 subunit mRNA were detected in the spinal cord at any developmental stages. The epsilon 4 subunit mRNA was distributed widely in the spinal cord during embryonic and early postnatal periods but decreased nearly to background levels by P21. In contrast to the differential distribution of the epsilon subunit mRNAs, the zeta 1 subunit mRNA was found ubiquitously at each developmental stage examined. These findings suggest that the molecular organization of the epsilon subunits may be different between the dorsal horn and the remaining regions in the mature spinal cord, which provides a molecular basis for functional heterogeneity of the NMDA receptor channel. Moreover, this spatial heterogeneity might be generated through drastic alterations in the subunit composition of the channel complex during spinal cord development.