The location of stem cells and the direction of colonocyte migration in the normal rat colonic crypt were investigated using a serial bromodeoxyuridine (BrdU) and [3H]thymidine labeling protocol. The results demonstrate that in the distal colon the stem cells are located in the crypt base and that cells migrate up toward the luminal surface. In the proximal colon, however, the stem cells are located in the midcrypt, and the colonocytes migrate in two directions, up toward the luminal surface and down toward the crypt base.