The shape of the main excretory duct (MED) of theMongolian gerbilsubmandibular gland and also the fine structure and endocytic ability of MED epithelial cells were examined. The neck was cut and sections of MED processed for light and electron microscopy. Following light microscopic observation, sections were read into a computer for three-dimensional analysis. Moreover, to examine the endocytic ability of MED epithelial cells, a cationized ferritin solution was infused into MED of various animals.<BR>Light microscopy and computer-graphics revealedthat the Mongolian gerbil submandibular MED had an ampulla within the submucosa under the oral cavity close to the caruncula sublingualis. The ampulla had a diameter about 5 times larger than that of the duct between the gland proper and the ampulla, and its anterior, lateral and posterior walls were surrounded by striated muscle fibers. Transmission and scanning electron microscopic observation showed that MED epithelium is a pseudostratified epithelium composed of type I, II and III cells. Type I cells had a number of short microvilli and microridge-like structures and several long microvilli on their broad free surfaces. Type II cells contained numbers of short microvilli on their narrow free surfaces. Cells of this type were observed in greater numbers in the ampulla than in the duct near the gland proper. Also, they had more vesicles and multivesicular bodies than the type I cells.Type III cells proved to be basal cells, positive for BrdU immunostaining but not for actin immunostaining. Cationized ferritin was recognized in the vesicles and multivesicular bodies of type I cells in addition to type II cells.<BR>These findings suggest that the ampulla temporarily stores saliva and that MED epithelium might be able to modify its protein component.