The distribution of collagen types I and III and elastin in the developing leg muscles were studied by immunohistochemistry in rat. From 0-day to 8-weeks old, the size of the gastrocnemius and plantaris muscles increased. The muscle connective tissue developed in the order of epimysium, perimysium and finally endomysium. The epimysium contained a considerable amount of collagen types I and III and some elastin in the neonates. These components in the epimysium remained almost unchanged in their distribution during development. The perimysium had little collagen type I and III or elastin at 0 day. Collagen type I and elastin slightly increased around 2 and 1 week, respectively, and returned to the previous levels. Collagen type III, however, increased and became abundant after 1 week. In the endomysium, the amounts of collagen type I and elastin were slight during postnatal growth, while collagen type III gradually increased after 2 weeks. The intramuscular tendons consistently showed intense reactivity for collagen type I and weak staining for elastin, whereas the staining for collagen type III decreased after 1 week and was finally restricted to the surface of intramuscular tendons. This study clearly demonstrated that the distribution of collagens, but not of elastin, significantly changed during development. The increase in collagen type III in the perimysium and endomysium, and its decrease in the intramuscular tendons probably reflect functional demands imposed on these connective tissues, i.e., shear forces in the former two and tensile loading in the latter.