The species-specific difference of the immunohistochemical localization of aquaporin-1 (AQP1) and aquaporin-4 (AQP4) was investigated in the cochleae of the 3 different species of rodents, including guinea pig, mouse and Mongolian gerbil. In the guinea pig cochlea, intense AQP1-like immunoreactivity was present in the type III fibrocytes in the spiral ligament and the mesenchymal cells just below the basilar membrane. Immunostaining was also found in some type IV fibrocytes in the spiral ligament, fibrocytes in the spiral limbus and mesenchymal cells lining the perilymphatic space against the bony otic capsule. In contrast, no remarkable immunostaining was found in the basilar membrane of the mouse cochlea. The medial part of the Reissner's membrane was positively immunostained with anti-AQP1 antibody only in the mouse cochlea. In the gerbil cochlea, AQP1-like immunoreactivity was weak compared with the other 2 species. AQP4 was found in the cochlear supporting cells, including Claudius cells, Hensen's cells and inner sulcus cells of the 3 rodent species. AQP4 was also expressed in some interdental cells of the spiral limbus. Weak immunoreactivity was also found in the root cells only in the upper turns of the guinea pig cochlea. In contrast, no detectable immunoreactivity was found in the root cells of the other 2 species. The results obtained in the present study provide the first evidence for the existence of the species differences in the expression of the AQP1 and AQP4 proteins in the rodent cochlea.