The kinetics of epithelial cell replacement in the descending colon of the mouse were studied using cell counts and other measurements in mice given a single injection of 3H-thymidine at 10:00 a.m. Cell migration occurs from the base of the crypts in the direction of the colonic lumen. The mean turnover time of epithelial cells as measured after the single injection of 3H-thymidine has been estimated at 4.85 days. Since this figure is in agreement with published data based on continuous infusion of the labeled precursor, it is concluded that the time selected for the analysis provides an index of proliferation which is representative of the mean proliferative activity of the epithelium. If each side of a crypt cut along its axis is referred to as a "crypt column," the mean number of cells per crypt column has been found to be 32.9; after a one-hour pulse labeling starting at 10:00 a.m., the mean number of labeled cells per column was 2.8; and the overall labeling index of the epithelium, 8.6%. The frequency of labeled cells in each cell position along the crypt column varies according to a Poisson distribution with a peak close to position 3. Presumably then, cell proliferation is taking place in a random manner and is, therefore, asynchronous, despite evidence to the contrary in the literature. The kinetic analysis supports the view that cell migration occurs along the crypt column in the direction of the colonic lumen at a velocity averaging 0.28 cell position per hour, or 4.48 microns per hour. The results support the view that vacuolated-columnar, mucous, and caveolated cells migrate jointly; whereas entero-endocrine cells migrate separately from these cell lines, except during the initial stages of the migration.