A major role of DNA-methyltransferase (MTase) is thought to be maintaining the DNA methylation profile through DNA replication. However, previous surveys of mRNA distribution in different tissues by Northern-blot analysis have shown unexpectedly high levels of expression of DNA-MTase mRNA in adult mouse brain, which consists mostly of slowly proliferating glial and nonproliferating neuronal cells. In order to identify cells expressing the gene in the brain, we performed an in situ hybridization analysis of mature brain as well as whole embryos of different ages. In addition to various embryonic tissues with active cell proliferation such as the ventricular neurogenic layer, hair follicle epithelia, thymus and epithelia of the base of intestinal villi, almost all mature neurons in brain of adult and even aged mice expressed DNA-MTase mRNA at substantial levels. No significant expression of the gene was detected in the white matter. These findings suggest some neuron-specific biological function of DNA methylation, unrelated to DNA replication.