The plain viscacha (Lagostomus maximus) has been used as an animal model because it has unusual anatomical–physiological characteristics in eutherian mammals. The aim of this work was to contribute to a better understanding of the digestion of L. maximus based on the description of the macroscopic structure of the stomach, and the histologic characterization and glycosylation pattern of the cardiac and pyloric regions. The stomach of L. maximus is monocavitary with an entirely glandular mucosa. This organ presented an acute angular incisura, so the cardiac and the pyloric regions got close together forming a “U” shaped stomach. The techniques applied put into evidence marked variations—according to the anatomical region analysed—in the histochemical profile of the mucous cells of the secreting epithelium, the gastric pits and glands. Moreover, the glycosylation pattern of mucous cells varied along the longitudinal axis of the cardiac and pyloric glands: differences between cells near the gastric pits (superficial zone of the glands) and those near the muscularis mucosae (deep zone of the glands) were observed. The information produced in the present work could be used as a relevant biological tool, contributing to the morphological bases for future anatomical and histophysiological analyses of the species.