Changes in myoproteins during development of rat skeletal muscle were investigated using two-dimensional gel electrophoresis. In M, soleus (SOL) which in adult, is composed predominantly of slow twitch fibers, fast type myosin light chains (fLC) were the major species and slow type light chains (sLC) were the minor species at birth. During development, the replacement rate of fLC to sLC sequentially occured so that LC patterns at 21 days postpartum were similar to adult where fLC were difficult to visualize. In contrast, M. extensor digitorum longus (EDL) always contained dominant fLC although sLC were found only for 5-9 days. LC 3 f became detectable at 5 days and gradually increased. In α-tropomyosin there were isozymes of fast and slow type based on difference in molecular weight, but not in β-tropo-myosin. Changes in isozymes of α-tropomyosin approximately corresponded with that in isozymes (fast and slow type) of LC in both EDL and SOL. During adult stage following birth, in EDL creatine kinase underwent a three-fold increase in molecular ratio to actin, whereas in SOL there was little change though increase took place transiently. These results suggest that with develoment many myoproteins change more dramatically in slow muscle than in fast muscle, and that transitions in LC isozymes and changes in distribution of histochemically typed muscle fibers may follow different time courses.