Nociceptive afferent signals from the orofacial area are transmitted to the trigeminal subnucleus caudalis (Vc) through the release of glutamate and/or substance P (SP). Although nociceptive transmission and/or modulating mechanisms are known to develop during the postnatal period, the specific developmental changes in nociception and/or modulation remain unclear. The present study examined postnatal changes in the spatial relationship between SP and its receptor, the NK1 receptor (NK1R), in the mouse Vc by immunohistochemistry and quantitative analysis. The medulla was removed from C57BL/6N mice (1, 2, 4, and 8 weeks of age) after perfusion and fixation, and cut horizontally at a thickness of 40 mum. The relative densities of SP- and NK1R-immunoreactive areas and their changes with age were assessed statistically. One- and 2-week-old mice showed relatively high densities of SP-positive structures in the marginal layer (Mar) and the deep part of the magnocellular layer (Mag). The SP distribution in the superficial Vc remained unchanged, but the density in the deep Mag gradually decreased with age, resulting in a complete loss after postnatal week 4. The NK1R-immunoreactivity exhibited a similar distribution pattern to that of SP, but the pattern remained unchanged during the postnatal period. Double-immunofluorescence staining for SP and NK1R demonstrated only moderate direct contact of SP-positive structures with NK1R in the superficial area. These separate distributions and the postnatal changes in SP and NK1R suggest the possibility of another nociceptive afferent transmission mechanism, that is, volume transmission, in the Vc other than synapse-mediated transmission.