Shiverer is an autosomal recessive mutant characterized by dysmyelination in the central nervous system. The myelin in the shiverer is devoid of the major dense line. The pattern of increase of 2',3'-cyclic nucleotide 3'-phosphohydrolase (CNPase) in the various parts of central nervous system during postnatal development was identical with that of controls. Subcellular fractionation of the brain from shiverer showed that the recovery of CNPase activity of the P2A was only 14% of the control, but those of P2B and P3 were several times higher than the controls. The specificity activity of CNPase of the purified myelin and P2A from shiverer did not differ significantly from that of the controls. The combined P2 and P3 (P2 + P3) fraction was subjected to linear sucrose density gradient (0.32--1.3 M) centrifugation. By measuring CNPase activities of the fractions obtained after centrifugation, shiverer showed two peaks of CNPase activities in the sucrose gradient, although that of the control showed a single peak. The main peak obtained from shiverer was found in the region of higher sucrose concentration than that of the control, while a small peak was found in the region of lower sucrose concentration.