MEF2D, a member of myocyte-specific enhancer binding factor 2 (MEF2) gene family, was shown by Northern blot hybridization to be strongly expressed in the head portion of mouse embryos at later stages of ontogenesis, in the cerebellum and the cerebrum of adult mice, in cultured cell lines of neuronal origin, and in skeletal and cardiac muscles. During ontogenesis, MEF2D transcripts were detected by in situ hybridization in the olfactory bulb, entorhinal cortex, pyriform cortex, and hippocampus, in Purkinje and granule cells, and in large neurons in both the ventral and dorsal horns of spinal cord. Adult mice continued to express MEF2D in these limited areas of the central nervous system. Thus, MEF2D seems to be involved in either the differentiation process or the function of these neurons.