The developmental patterns of membrane-bound carbonic anhydrase (MBCA) and glial fibrillary acid protein (GFA) were analyzed in high speed pellets from subcortical regions of quaking and control mice at ages ranging from 10 to 42 days. In control, MBCA was found to increase from approximately 1.5 unit/mg protein at 10 days to about 11 units/mg protein at 42 days. The percent increase was greatest between days 11 and 18 where the activity increased 2.5-fold while the largest absolute weekly increment was in the fourth postnatal week. By contrast, the activity in quaking mice was 1.0 units/mg protein at day 10 and increased to only 5 units/mg protein by 42 days. The activity, although reduced at 10 days, appeared to increase in parallel with the controls between day 11 and 18; however, the large increment in activity observed in the controls during the fourth week was greatly reduced in the mutant. The above findings were also observed when immune reactive material was determined by immuno-blotting using antisera raised against MBCA isolated from brain. GFA was also estimated using an immuno-blotting procedure. In contrast to previous studies on soluble GFA, we found GFA to increase about 5-fold in the third postnatal week and level off thereafter. The Q/C ratio was 1.5 at 10 days, but, although GFA increased over 5-fold between 11 and 42 days, the absolute difference between quaking and control increased very little. These data on GFA and MBCA suggest that a defect in the expression of oligodendroglial plasma membrane proteins may be expressed early in development which may correlate with a small but significant gliosis and/or an increase in GFA synthesis.