Myelin basic proteins (MBPs) are major proteins of central nervous system (CNS) myelin, where they facilitate the apposition of cytoplasmic faces of myelin lamellae. Myelin-bearing oligodendrocytes transport MBP mRNA to myelin, where newly translated protein is directly inserted into the myelin sheath. An apparent absence of MBPs in oligodendrocyte perikarya has suggested that MBP localized to the soma is translationally inert. We now demonstrate by confocal immunofluorescence microscopy that not only are MBPs present in the majority of oligodendrocyte perikarya but oligodendrocytes are heterogeneous with respect to their localization of MBPs; MBPs are concentrated in some cells at the plasmalemma and distributed in others throughout the cytoplasm and, surprisingly, the nucleus. MBPs are present in the nuclei of over half of oligodendrocytes in the adult, but in almost all MBP+oligodendrocytes during myelinogenesis. Transport of MBPs into nuclei appears to be a regulated process since some cells exhibit robust MBP accumulation in their cytoplasm but exclude MBPs from their nuclei. We show that oligodendrocyte nuclei contain all four major MBP isoforms, but that in transgenic mice, the epitope-tagged 14 kD MBP isoform preferentially segregates to the plasmalemma. Our data demonstrate that oligodendrocytes are not required to exclude MBPs from their perikarya and suggest that MBPs have a specific function in the oligodendrocyte perikarya and nucleus.