Decorin, a small leucine-rich proteoglycans, plays an important role in tissue morphogenesis through the regulation of collagen fibrillogenesis and the modulation of some growth factors. Our recent study has shown that decorin binds to myostatin, a negative regulator of skeletal muscle mass, and modulates its inhibitory action to myogenic cell growth in vitro. However, it still remains unclear whether decorin binds to myostatin in vivo during the development of skeletal muscle. To clarify this, we investigated the spatiotemporal expression of decorin and myostatin in rat skeletal muscle by RT-PCR and immunohistochemistry. Decorin mRNA abundance in fetal skeletal muscle was significantly higher than those in neonates and adults (P<0.05). Decorin mRNA expression decreased drastically at birth, and thereafter gradually up to 9 weeks of age. The mRNA expression pattern of myostatin was quite similar to that of decorin during prenatal and postnatal development of rat skeletal muscle. Immunohistochemical analysis demonstrated that myostatin was located in the muscle fibers, and that decorin was located in the periphery of muscle fibers in fetal rat skeletal muscle. Taken together with our previous data, these results suggest that decorin binds myostatin and sequesters it in the ECM during the development of rat skeletal.