Cell senescence is seen in many types of differentiated cells but age changes in stem cells have not previously been clearly demonstrated. Changes in stem cells may be of great importance for the ageing process, because any decline with age in the numbers and functional integrity of stem cells can lead to progressive deterioration of function and of proliferative homeostasis in tissues. Stem cells of the murine small intestine provide an excellent model system because these cells occupy a well-defined position near the base of the crypts of Lieberkühn. We examined mice aged between 5 and 32 months and found age-related alterations in the histology of the small intestine and in the apoptotic response of stem cells to low-dose irradiation. Apoptosis in the crypts is concentrated around the stem cell position and can be markedly elevated by exposure to radiation or cytotoxic agents, suggesting that “suicide” of damaged stem cells may be an important system for long-term tissue maintenance. Animals aged 5, 15, 18, and 29 months were exposed to either 1 or 8 Gy gamma irradiation. A twofold increase in the level of apoptosis was seen following 1 Gy gamma irradiation in the 29-month-old animals, compared to the young and middle-age groups. After 8 Gy irradiation the level of apoptosis in all age groups was high and the age effect less pronounced. The data suggest that stem cells do undergo some functional alteration with age.