Abstract: In the present study,the effects of postnatal development on the number and distribution of skeletal muscle fibers of different types in hind leg of rat and rabbit were studied.The soleus muscles of rats aged 2 days,2,4,6,8,and 10 weeks (body weight 10,32,95,190,280,and 320 g),and rabbits aged 2 days,2,4,8,12,16,20,and 24 weeks (body weight 100,220,400,750,1 200,1 600,2 100,and 2 500 g) were stained with succinic dehydrogenase.With an image analysis system,the X-Y coordinates of fibers were used to analyze the growth-related changes.The results of present study showed that three types of fibers were found in the soleus muscles of rat and rabbit,I.e.,type Ⅰ (slow oxidative),ⅡX (fast oxidative),and ⅡA (fast oxidative glycolytic).The type Ⅰ fibers were present throughout the muscle that had a uniform distribution and tended to increase in number with aging.Type ⅡX fibers were scattered throughout the muscle and decreased markedly in number with aging.Type ⅡA fibers were located at the central and deep regions,and showed a little or no change in number and distribution with aging.While be of age,type ⅡA and ⅡX fibers became restricted to the superficial region.No type ⅡB fibers were detected.Type ⅡA fibers had the largest diameter,type Ⅰ intermediate and type ⅡX the smallest.Mean cross-sectional area of each type fibers of rabbits was larger than that of rats.The present results indicate that the number and distribution of muscle fibers of different types in hind limb of rat and rabbit change with the process of postnatal growth.