The effect of different levels of activity on fiber types, capillaries and enzymes of gastrocnemius and soleus muscles was studied in two groups of cats. The first group was successfully kept in a large room, exercised on a treadmill 15 min daily 5 days per week and kept in individual small cages. Each period lasted 6 weeks. A muscle biopsy was taken after each period. The second group was formed by cats that were caged for over 20 months. In the group caged for over 20 months, gastrocnemius muscle showed higher IIB and lower I fiber type proportion. Fiber cross-sectional area was not different in any condition. All capillary measurements were significantly lower in gastrocnemius muscle of long-term caged cats, and capillaries per mm2 were lower in soleus muscle of these cats. Exercise increased capillary/fiber in soleus muscle but subsequent caging did not reduced it. In soleus muscle, beta-hydroxy-acyl-CoA dehydrogenase levels decreased after the cage period and hexokinase levels increased after the exercise and decreased after the cage period. In conclusion, different levels of activity for short time produced enzyme changes in soleus muscle, whereas long-term inactivity changed fiber type proportion in gastrocnemius muscle and reduced capillary supply.