In situ hybridization in the cochlea has suggested that pendrin mRNA is expressed in cells that reside immediately beneath the spiral prominence on the lateral wall of the external sulcus [8]. To determine the location of pendrin protein expression, we performed confocal immunocytochemistry on cryosections prepared from temporal bones of normal (Slc26a4+/+) mice using an established antibody [3]. Staining was absent when the primary antibody was pre-absorbed with the antigenic peptide (data not shown). Strong expression of pendrin was observed not only in outer sulcus epithelial cells, as predicted from in situ hybridization data, but also in root cells, in apical membranes of spiral prominence surface epithelial cells and in apical membranes of spindle-shaped cells that are part of stria vascularis (Fig. 1a,1b,1c). The presence of pendrin in spindle-shaped cells suggests that these cells secrete HCO3 - into endolymph. Pendrin-mediated HCO3 - transport has previously been shown in the kidney [9]. HCO3 - in the cochlea may be generated from CO2 by carbonic anhydrase located in strial intermediate cells [18,19]. CO2 may be supplied by the metabolically highly active stria marginal cells. It is conceivable that pendrin dysfunction interrupts HCO3 - secretion and leads to an accumulation of HCO3 - in stria vascularis. Preliminary data (Wangemann et al., unpublished) support a role for pendrin in HCO3 - secretion into endolymph.