We investigated the onset of expression of the myelin/oligodendrocyte glycoprotein (MOG) mRNA and protein in the developing mouse central nervous system. In situ hybridization on brain sections at different stages of embryonic and postnatal development showed that MOG transcripts were first detected at birth in the medulla oblongata. During the first week after birth, cells expressing MOG mRNA were located in the ventral longitudinal funiculus. During the second postnatal week, the pattern of MOG mRNA expression extended rostrally to the mid-forebrain regions and reached completion by the beginning of the third week. MOG transcription was delayed by several days with respect to myelin basic protein (MBP), and it appeared that while the MBP probe labeled both non-myelinating and myelinating oligodendrocytes, only the latter were MOG-positive. In vitro, immunocytochemical analysis of MOG protein expression, performed on myelinating cultures derived from mouse brain embryos at 15 days of gestation, confirmed the strict restriction of MOG expression to myelinating oligodendrocytes. In particular, oligodendrocytes lining up their processes along axons, but not yet having started to deposit a myelin sheath, were still MOG negative. However, in the same cultures, pseudo-myelinating oligodendrocytes (i.e., cells not associated with neurites, but forming whorls of myelin-like figures) were MOG positive. Similarly, rat CG4 cells, an oligodendrocyte-like cell line, expressed MOG only after they had extended sheet-like processes, which suggested that the activation of MOG transcription depends more on an intrinsic oligodendroglial maturation program of myelination than on a neuronal signal.