The aging of the cells depends on their mitotic activity, which is particularly evident in multicellular organisms. By way of histoautoradiography we studied the cell kinetics on a total of 244 Wistar rats aged 6 days, 6 weeks, 6, 12, 23 and 28 months respectively, with these studies being centred on the mucosa of the small intestine since this mucosa is one of those tissues with the highest cell metabolism. We succeeded in demonstrating that the regeneration rate of cells per hour in the crypts of the small intestine and the migration speed of the enterocytes differ in young and old individuals and that the intermitotic cells have age-dependent properties as well. In addition, we could prove that intermitotic cells too have a non-growth fraction which, at an advanced age, decreases only a little, although significantly in terms of statistics. For the easily vulnerable crypt epithelium it is a reserve capacity and can be included in the proliferating pool, if necessary.