The distribution of carbonic anhydrase isoenzymes (CA) was analyzed in the developing endolymphatic sac (ES) of the mouse embryo and human fetus using immunohistochemical method. The primordial ES epithelium was labelled with CA I and CA II, but was weakly labelled with CA III and CA V. In the thirteenth and fifteenth gestational day (GD) mice, the ES epithelium was positive for CA I and CA II. After seventeenth GD, the ES epithelium was however weakly positive for CA I and CA II. In the 11 and 12 week old human fetus, the ES epithelium was strongly labelled with CA I and CA II. In the 16 week old human fetus, the ES epithelium was however weakly positive for CA I and CA II. These results suggest that the fetal ES has an activity of CA and plays a role in the otoconial formation especially in the early stage during evolution.
