Pathological observations were carried out on 100 male rats of Sprague-Dawley strain obtained from 5 outbreaks of sialodacryoadenitis (SDA). In accordance with clinical and macroscopic findings, principal microscopic changes were found in the salivary (submaxillary and parotid) and Harderian glands. Lesions observed in these different glands were essentially similar to each other, and characterized by degenerative changes in the epithelial cells of the ducts and acini associated with marked and extensive serous inflammation in the interstice. Furthermore, squamous metaplasia of the ductal epithelia was a common feature to these glands in the late stage of the disease. Some differences, however, were detected between the changes of the salivary glands and those of the Harderian glands. Firstly, the ductal epithelial cells were affected at first and the acinous epithelial cells were damaged only secondarily in the active inflammatory process in the salivary glands, while both ductal and acinous epithelial cells were involved equally in the primary lesion in the Harderian glands. Secondly, the increase in connective tissue elements in the reparatory process was extremely marked in the Harderian glands as compared with the salivary glands. Consequently, in the lesion of Harderian glands in the late stage, original construction was almost indiscernible, and the ducts and acini were only recognized as disseminated islets in actively proliferated connective tissue elements.