For most vertebrate organs and tissues, the majority of development occurs during embryogenesis, and postnatal changes are primarily concerned with growth. The central nervous system is unusual in that a considerable amount of morphological development, cell differentiation and acquisition of function, takes place during postnatal development. As yet, the molecular mechanisms underlying these complex developmental processes are not well understood. In order to identify markers for these developmental processes, we have analyzed the expression profiles, during postnatal murine brain development, of approximately 25,000 transcripts. This analysis, performed at day 1, day 10, day 20 and day 42 of postnatal development, identified a large number of developmentally regulated genes which we have assigned into three broad expression categories.