Calmodulin (CaM) binding sites, which are available and unoccupied by CaM at the time of fixation, were analyzed in the guinea pig endolymphatic sac and stria vascularis using CaM conjugated tetramethyl rhodamine isothiocyanate. The endolymphatic sac epithelium and stria vascularis both showed strong fluorescence. Weak CaM binding sites were seen in the endolymphatic sac in the 12-week-old human fetus. In contrast, the fluorescent reaction in the stria vascularis and endolymphatic sac in 14- and 15-week-old human fetuses was strong. These results suggest that CaM may play an important role for the maintenance of ionic stability in the endolymph.