The effects of ageing and of exercise on muscle mass, fiber cross-sectional area, and fiber type composition of a weight-bearing muscle, the soleus and a non-weight-bearing muscle, the extensor digitorum longus (EDL) were investigated in female Long-Evans rats. The animals were exercised by means of voluntary wheel running beginning at 4 months. Runners and sedentary controls were studied at 9 months and 27 months of age. In sedentary rats, the soleus muscle weighed 26% less, and the EDL weighed 19% less at age 27 months, than at 9 months. This decline in muscle mass was accounted for by a similar decrease in muscle fiber cross-sectional area. The wheel running resulted in significant hypertrophy of the soleus in both 9- and 27-month-old rats; as a consequence the 27-month-old runners had larger soleus muscles than the 9-month-old sedentary rats. The running did not prevent atrophy of the EDL in the old rats, but did increase the proportion of type IIa fibers. The exercise also increased the number of capillaries per fiber in the soleus muscles of both young and old rats. In conclusion, the finding that wheel running prevented atrophy with ageing of the weight-bearing soleus but not of the non-weight-bearing EDL emphasizes the specificity of exercise, and shows that exercise-induced muscle hypertrophy can be maintained in old age by appropriate exercise.