The kinetics of cell proliferation, migration and renewal in the pyloric mucosa of golden hamsters were studied by flash, cumulative and pulse labelling autoradiography following 3H-thymidine injections. By flash labelling autoradiography, it was shown that the labelled epithelial cells are exclusively confined to a zone several cells wide in the region of the isthmus between the gastric pits and the pyloric glands. In the cumulative and pulse labelling experiments, this cell proliferation in the isthmus region was shown to be for replacement of both the surface epithelial and the glandular cells. The surface epithelial cells of the pyloric mucosa arising in the upper portion of the isthmus come to line the pits and the surface, and are sloughed off into the gastric lumen within a week. The mucin-containing glandular cells, which arise more deeply in the isthmus region, migrate downwards and are apparently lost at the deepest level of the glands. The life span of the mucin-containing glandular cells was estimated at about 14 days. This cell type appears to undergo renewal of the "first produced, first lost" pipe line variety. However, a small number of glandular cells was found to survive for more than 20 days (up to 30 days), suggesting the existence of a sub-population of cells with different kinetics in the pyrloric glands.