Homozygous quaking and normal control littermate mice were injected intracerebrally with [3H]leucine at 19 days of age. The animals were sacrificed after 1 h and after 6 days. The proteolipid protein (PLP) and intermediate protein (IP) were extracted from whole brain by chloroform-methanol (2:1) and resolved by sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE). One hour postinjection the labeling of total protein in quaking brain was the same as the control and the radioactivity of PLP and IP in quaking was approximately 35% of the control. Six days after precursor administration the radioactivity of the total protein decreased significantly in both groups and to the same extent. However, the labeling of PLP and IP more than doubled in the control, while it decreased by half in the quaking brain. The results indicate that there is an increased turnover rate of PLP and IP in quaking brain.