In the replacing tissues of the body, namely the bone marrow, testis, and the surface epithelia with their appendages, cell replacement would appear to be achieved using an hierarchically organized proliferative compartment with relatively few ultimate stem cells producing dividing transit cells which eventually differentiate and mature into the functional cells of the tissue. The cell cycle times of the various constituents of the hierarchy differ, and the stem cells apparently have a longer cell cycle than the transit cells. There may be variations in the cell cycle as cells pass through the transit population in some cases, e.g. in the bone marrow, while in others the cycle time remains fairly constant, e.g. in the testis. The difference in the cell cycle time between stem cells and transit cells is not completely unequivocal, and there is little or no difference in cycle time in the epithelium on the dorsal surface of the tongue while in other cases the experimental evidence for long stem-cell cycles is somewhat imprecise. However, the epithelium in the small intestine and the spermatogonia in the testis have been fairly extensively studied and here the evidence clearly shows a lengthening of the cell cycle as more primitive cells are considered.