There are contradictory reports on the effect of the estrous cycle on cell proliferation in the intestinal epithelium. We reexamined estrous-cycle-related changes in the epithelium by using flow cytometry and Coulter particle counting. We found significant estrous-cycle-related variations in both the distribution of epithelial cells through the cell cycle as well as the number of epithelial cells per unit area in mouse jejunal and colonic epithelium.