The regional and cellular distribution of the high activity carbonic anhydrase isoenzyme (CA C or CA II) in the mouse nervous system was investigated by an indirect immunoperoxidase (peroxidase-antiperoxidase) method using cross-reactive antibodies prepared against human CA C. In the mature brain an overall strong CA C specific reactivity was revealed in the heavily myelinated nerve tracts, the main immunostaining originating from small, intensively reacting cells interpreted as oligodendrocytes and from the myelin sheaths. An obvious straining was also revealed in the choroid plexus cells, especially in their free borders, and in the erythrocytes of the blood vessels, while the glial cells of the retina similarly exhibited a strong reaction. In the developing brain, CA C was absent shortly after birth, but achieved almost the mature pattern of distribution within about 3 weeks. In the spinal cord most of the positive cells were found in the grey matter, their processes projecting towards the white matter. No reaction was obtained in the sciatic nerve fibers or in the neuronal or satellite cells of the coeliac ganglion.