Brush cells (BCs) are relatively rare cells that are sparsely distributed throughout the mammalian digestive and respiratory systems. BCs have been identified in the rodent large intestine, but these cells have not been characterized by immunocytochemistry or electron microscopy. We previously demonstrated that rat bile duct BCs had strong immunoreactivity for six proteins that function in HCO ₃ ⁻ secretion and thus assumed that BCs secrete NaHCO₃. It is well known that the gastrointestinal (GI) tract secretes NaHCO₃, but it is not known whether BCs of the GI tract also express proteins related to HCO ₃ ⁻ secretion. Thus, in the present study, using double immunostaining for cytokeratin 18, a specific marker for BCs, we investigated protein expression in BCs from the rodent GI tract. We show that BCs from the GI tract express six proteins related to HCO ₃ ⁻ secretion: cystic fibrosis transmembrane conductance regulator (CFTR), Cl⁻/HCO ₃ ⁻ exchanger, Na⁺/HCO ₃ ⁻ cotransporter, carbonic anhydrase II, Na⁺/H⁺ exchanger (NHE) 1, and NHE3. These results suggest that BCs from the GI tract secrete NaHCO₃. In addition, we examined BCs from the rat cecum using electron microscopy (EM). Transmission EM (TEM) showed that BCs have long microvilli, a well-developed tubulovesicular system, and an abundant cytoskeleton. Scanning EM revealed that BCs were scattered on the luminal surface of the cecum and had numerous long microvilli.