In the opossum the fundic area of the gastric mucosa develops first, and oxyntic glands show early division into gastric pits and glandular components which then grow simultaneously. At 40 d postnatum the oxyntic glands are longer than the gastric pits but in the pyloric glands the pits still are longer than the glands and it is not until the 73rd postnatal day, that pyloric glands are longer than their pits. Development of the cardiac glands is delayed even more and the length of the gland does not surpass that of the pit until about 95 d postnatum. Argyrophil cells are concentrated in the cardia and fundus at birth and during early postnatal life, but the population of these cells shifts to the pylorus in the adult. Argyrophil cells of the cardiac, oxyntic and pyloric glands are concentrated in glandular regions with the highest mitotic activity.