The non-diseased portions of the antral mucosa of patients suffering from gastric cancer or ulcer were biopsied. The biopsy specimens were then labelled with 3H-thymidine in vitro, and distribution of the labelled epithelial cells in the normal pyloric and in the intestinalized mucosa was studied with autoradiography, and modes of histogenesis and spreading of the intestinal metaplasia were studied, and kinetic characteristics of the intestinalized mucosa were discussed. In the normal pyloric mucosa, the labelled cells were confined to the isthmus region (the middle one-third level of the mucosa), indicating that the surface epithelial and the pyloric glandular cells are normally replaced from the isthmus region. On the other hand, a zone of the labelled cells was found at the lower one-third level in the intestinalized mucosa. The absorptive and the goblet cells in the intestinalized mucosa appear to be renewed by about 70 hours in a fashion similar to that of the small intestine. Microscopic and autoradiographic analysis of the antral mucosa in the course of intestinalization indicates that the intestinal metaplasia begins in the isthmus region of the pyloric glandular tubules of an intact mucosa unaffected by gross injury through transformation of the generative cells from a pyloric to an intestinal pattern. This permits the pyloric lining cells to be replaced with intestinal villous cells and also permits the generative cell zone of the intestinal tubules to shift from the isthmus to the base of the gland until the process is complete. The downward shift of the intestinal tubules occurs in a framework of one of the branched pyloric glands and other glands disappear, resulting in a change of mucosal architectures of the antrum from a branched to a simple tubular gland. The intestinal metaplasia spreads in the mucosa through multi-focal (and sporadical) transformation of the neck generative cells in individual glandular tubules.