Objective To analyse and study the distribution of muscle fiber and the composition of myosin heavy chain isoforms(MHCs) in each muscular subvolume of the lateral gastrocnemius of adult SD rats. Methods Succinate dehydrogenase(SDH) staining was applied to analyse the proportions of muscle fibers of four types and sodium dodecyl sulphatepolyacrylamide gel electrophoresis(SDS-PAGE) was also used to measure myosin heavy chain isoforms(MHCs). Results The proportions of types Ⅰ(SO), ⅡX(FO), ⅡA(FOG) and ⅡB(FG) muscle fibers in medial subvolume were (10.2±4.1)%、(2.8±2.1)%、(25.7±9.6)%、(61.3±10.1)% respectively, (15.9±8.2)%、(6.1±2.6)%、(22.8±11.5)%、(55.2±12.6)% in lateral superficial subvolume, (21.3±9.2)%、(9.8±2..5)%、(18.5±8.7)%、(50.4±10.9)% in lateral profound subvolume. The distribution of muscle fibers of four types was of a crisscross and mosaic pattern. Type ⅡX muscle fiber had the smallest transverse area and type ⅡB the biggest, with a medium and a smaller transverse areas for types ⅡA and Ⅰ respe