To investigate whether or not retardation of body weight increase by reduced food intake could change enzyme activities in the soleus and extensor digitorum longus (EDL) muscles during growth, rats were divided into three groups. Group 1 included rats aged 5, 7, 9, 11 weeks fed ad libitum. Group 2 included rats aged 5, 7, 9, 11 weeks fed a restricted diet. These animals weighed as much as the 5-week-old rats in group 1. Group 3 included four subgroups of 11-week-old rats. Their body weights were adjusted to the weights of rats 5, 7, 9 and 11 weeks old in group 1 by restriction of food intake. Succinate dehydrogenase (SDH) activity in the soleus muscle increased significantly with growth. Though a similar change was not observed with age in group 2, SDH activity in the soleus muscle in group 3 was enhanced with increasing body weight. Phosphofructokinase (PFK) activity in the soleus muscle decreased with growth in groups 1 and 2, but this change was not statistically significant. In group 3, PFK activity in the soleus muscle did not fluctuate with increasing body weight. In groups 1 and 2 there was a significant correlation between PFK activity and percentage area of type I fibers in the soleus muscle (r=-0.63, p<0.01, r=-0.55, p<0.01, respectively), but no significant relationship between them was evident in group 3. PFK activity in the EDL muscle increased significantly with growth in groups 1 and 2, but did not change with increasing body weight in group 3. It is suggested that the increase of SDH activity in the soleus muscle between the ages of 5 and 11 weeks is influenced primarily by changes in body weight, but that the changes of PFK in the soleus and EDL muscles are not modified by this factor.