aB-Crystallin is one of two subunits of multimeric protein a-crystallin (aA and aB), which is a major component of the lens. As a molecular chaperon and heat shock protein, aB-Crystallin prevents false protein folding and has anti-apoptotic properties. In this study, aB-crystallin expression was investigated in the central nervous system (CNS) using the tissues collected from the cerebrum, cerebellum, olfactory bulb, pituitary gland, medulla oblongata, cornu ammonis, hypothalamus and cervical spinal cord. Except in the pituitary, aB-crystallin immunolocalized in glial cells in all nervous tissues investigated. No immunoreactivity was observed in neurons. In double staining of aB-crystallin and 2',3'-cyclic nucleotide 3'-phosphodiesterase (CNPase), the immunoreactive glial cells were determined to be CNPase positive as oligodendrocytes. Immunoreactive oligodendrocytes located mainly the in the white matter and the layers of the gray matter adjacent to the white matter. However, the spinal cord had numerous aB-kristallin positive oligodendrocytes both in the white and gray matters. In the white matter of the spinal cord, a limited number of glial cells were positive for aB-crystallin, but negative for CNPase. aB-Crystallin immunoreactivity was weak in cytoplasm but very light in the nucleus. As observed in oligodendrocytes of normal dog CNS tissues, aB-crystallin may have roles in anti-apoptotic process of oligodendrocytes and prevention false folding of proteins such as myelin basic protein, which is synthesized intensively by oligodendrocytes.