The architecture of the pure mucous units of the pyloric antrum was investigated in 3- to 4-month-old CD1 mice. Units were serially cut in cross section and stained by a method combining the periodic acid-Schiff sequence, a modified Grimelius's silver nitrate procedure, and Regaud's hematoxylin. A total of 195 units were then reconstructed. Of these, six were cast in polyester resin and 189 were two-dimensionally reproduced on graph paper. The reconstructions showed antral units to be divided among three main classes. The first class, which contained 32% of the units, consisted of fingerlike tubules referred to as "singlets." Three types of singlets were observed. The first or type A, which represented 76% of the singlets, was divisible into three successive portions: a pit (foveola) opening onto the mucosal surface and lined by mucous cells referred to as pit cells, an isthmus continuous with the pit and containing immature proliferative mucous cells, and a gland forming the blind end of the tubule and lined by mucous cells referred to as gland cells. Type B (14% of singlets) was similar to type A except that its gland was forked. Type C (10% of singlets) differed by the absence of a gland. The units of the second class, which contained 53% of the total number, were joined together along part of their length and were named "multiplets." Most of them (90%) were organized into clusters of two, and 10% into clusters of three. In the joined portion, the epithelial cells of the adjacent units were in contact through junctional complexes and, therefore, were not separated by basement membranes. Otherwise the units showed the same component parts as in singlets. Also, as in singlets, the majority of the units were type A and a few were type B or C. The units of the third class, or "intermediates," consisted of tubules which exhibited a branching process. This process was of variable length but could include gland, isthmus, and sometimes pit. Thus, the process duplicated a varying proportion of the unit. In conclusion, the pure mucous units of the antrum exhibit various patterns which have been designated singlet, multiplet, or intermediate. It is proposed that these three patterns are related and represent temporal differences in the duplication and production of new units. Based on this assumption, a model has been elaborated to depict the likely sequence in the proliferation of pure mucous units. It is proposed that this proliferation takes place in the antrum of young adult mice.