In the colony of Sprague-Dawley (SD) strain, we found that there were rats expressing a mutant AQP5, which has a point mutation at nt 308 (G308A), leading to a replacement of 103Gly with 103Asp in the 3rd transmembrane domain. The mutant molecule scarcely expressed in the acinar cells, probably because of ineffective trafficking. The mutant molecule, however, showed normal water permeability when assessed by the oocyte system.