The brush cells (BC) of the mouse gallbladder were studied using light and electron microscopy (transmission and scanning) to determine their shape and distribution. Specimens were fixed in glutaraldehyde and postfixed in ferrocyanide-reduced osmium tetroxide. BC selectively stained with toluidine blue could be identified by means of light microscopy and subsequently studied in serial semithin and ultrathin sections. The results revealed that the shape of the BC is flask-like. A slender, occasionally branched cytoplasmic process emerges from the bulk cell body and extends through the basal region of neighboring epithelial elements to the basement membrane. Examination of the entire gallbladder epithelial surface by scanning electron microscopy revealed that the BC are numerous in the neck region of the organ but only scanty or even absent in wide areas of the corpus region. Their number increases again in the fundic region. These results demonstrate a preferential regional distribution of BC in the gallbladder, which is discussed in relation to a possible functional significance of the BC.