The recognition molecule myelin-associated glycoprotein is expressed by oligodendrocytes, the myelinating cells of the central nervous system. The myelin-associated glycoprotein gene gives rise to two alternatively spliced transcript variants ("early" and "late" message) which are developmentally regulated. In this study, using mice, we investigated whether both transcripts can be expressed in an individual oligodendrocyte or whether different oligodendrocyte populations exist expressing either one or the other myelin-associated glycoprotein messenger RNA. For this purpose the cytoplasmic RNA content of single oligodendrocytes derived either from cultures of embryonic mouse brain or from the corpus callosum murine slice preparation was harvested during patch-clamping in the whole-cell recording mode by applying negative pressure to the patch pipette. After reverse transcription, cDNA fragments were amplified by the polymerase chain reaction and analysed by agarose gel electrophoresis and restriction enzyme maps. Expression of myelin-associated glycoprotein transcripts could first be detected in those oligodendrocytes which already had acquired a more mature developmental stage. This stage could electrophysiologically be characterized by the dominance of passive K+ currents. In addition to oligodendrocytes expressing only the late or the early transcript, many cells were found expressing simultaneously both transcripts with varying levels. The myelin-associated glycoprotein transcript expression is therefore found to be developmentally regulated at a stage when oligodendrocytes have already acquired the channel properties of the adult.