In rats at the postnatal week 3 (P3W), enkephalin-immunoreactivity was detected in subsets of principal ganglion cells in the superior cervical ganglion and nerve fibers within the submandibular gland, one of the targets for the ganglion, whereas it disappeared from them at P8W. Enkephalin-immunoreactive ganglion cells and intraglandular fibers were detected again after the colchicine-pretreatment at P8W. By in situ hybridization the population density of ganglion cells expressing mRNA for preproenkephalin and the expression intensity were similar in both ganglia at P3W and P8W. These findings suggest that the post-transcriptional down-regulation of gene expression for preproenkephalin is involved in the disappearance of enkephalin-immunoreactivity in the adult ganglion.