Myelin was isolated from six regions of the central nervous system of quaking and littermate control mice. The yield of myelin from the mutant animals ranged from 2% of the control level in the most rostral parts of the central nervous system to 16% in the spinal cord, with an intermediate value (10%) for the optic nerves; these results are in close agreement with the data of others obtained by quantitative electron microscopy. Electrophoretic analyses of myelin from the telencephalon, diencephalon, brain stem and spinal cord revealed a marked deficit of proteolipid and small basic proteins in all fractions from quaking mice; however, neither of these proteins exhibited clear rostro-caudal trends with respect to the ratio of protein contents between quaking and control mice. An hypothesis is proposed to account for the regional variations in the mutant's myelin deficit; it is suggested that the quaking mutation induces an arrest in myelinogenesis through a mechanism which is common to both oligodendroglial and Schwann cells.