Three age groups (6, 22 and 27 months) of CWI mice (female, outbred strain) were subjected to a physical training program for up to 5 weeks. During the training period the body weight remained constant in all groups. Hind leg muscle mass increased in response to the exercise in 6 and 22 month old mice but decreased in the senescent (27 months) animals. Physical exercise also increased the key enzyme creatine kinase in the 6 months group and to a lesser degree in the 22 months group. By contrast, senile animals showed a progressive loss of the enzyme throughout the training period. Similar changes - though less-pronounced - were seen with regard to soluble protein and to total DNA. The data indicate that old animals (22 months) are still able to adapt adequately to physical training while senile mice (27 months) respond paradoxically by a negative adaptation to a physical challenge.