The present study has been undertaken aiming at the elucidation of the effect of growth and sex-related difference on metabolic properties of skeletal muscle in rat hindlimb. The slow soleus (SOL)and the fast extensor digitorum longus (EDL) muscles were obtained from Wistar strain rats aged from 3 to 75 weeks after birth. The results are summarized as follows: 1. The male rats, after 6 weeks of age, showed significantly higher increase in the body weight than the female. 2. The muscle weight gradually increased until 25 or 30 weeks of age in both sexes. The muscle weights of the SOL and EDL in female were always smaller than those in male at any period of age. 3. The glycolytic enzyme activity, obtained biochemically, in the EDL rised slowly with growth, but not in the SOL. The mean activity of oxidative enzyme in the SOL dropped with growth. There was a significant difference in oxidative enzyme activity between SOL and EDL of younger rat (up to 12 weeks of age). Beyound 12 or 15 weeks of age, however, the oxidative enzyme activity in the SOL dropped to the level of that obtained in EDL. 4. In the rats younger than l0 to 20 weeks, the glycolytic and oxidative enzyme activities of both muscles were greater in female rats than in ma1e. This inclination was reversed after the age of 20 weeks. It is concluded that the properties of glycolytic and oxidative enzyme activities in rat skeletal muscle are influenced by growth. A sex-related variation in the enzyme activity exists especially in the EDL muscle.