Laminin is an extracellular matrix-associated protein, which is largely present in the basement membranes of the human placenta, striated muscle and Schwann cell. Laminin has been proposed to promote attachment, spread, motility, growth and development of tissues or cells. In this study, we investigated the formation, localization and migration of the laminin in developing rectus femoris muscle of fetal and newborn rats. Experimental animals, fetal or newborn rats (Sprague-Dawley strain), were divided into 8 groups (14 day-old, 16 day-old, 18 day-old, 20 day-old and 22 day-old fetal rats, 1 day-old, 3 day-old, 5 day-old and 7 day-old newborn rats). The specimens of each group were prepared for detection of the laminin by immuno- histological and immunogold electron microscopic methods. 1. The primitive rectus femoris muscle of 14 day-old fetal rats consisted of mesenchymal cells and myoblasts. Laminin stained with a few gold particles were observed in the cytoplasm of myoblasts and rough endoplasmic reticulum of mesenchymal cells. 2. In the rectus femoris muscle of 16 day-old fetal rats, laminin was strongly expressed in myoblasts and mesenchymal cells. Gold particles were distributed in the cytoplasm of myoblasts and mesenchymal cells. 3. In the rectus femoris muscle of 18 and 20 day-old fetal rats, strong laminin immune reactions were observed in the basement membrane of muscle cells. A few gold particles distributed on the basal lamina of muscle cells and in the RER of fibroblasts. 4. In the rectus femoris muscle of 22 day-old fetal rats and 1 day-old newborn rats, strong laminin immune reactions were seen in the basement membrane of muscle cells. Numerous gold particles were located on the basal lamina of muscle cells and the RER of fibroblasts. 5. In the rectus femoris muscle of 3 day, 5 day and 7 day-old newborn rats, moderate laminin immune reactions were detected in the basement membrane of the muscle cell. A few gold particles were located on the basal laminae of muscle cells and RER of fibroblasts. These results suggest that the distribution of laminin in the rectus femoris muscle is related to myogenicity. Laminin seemed to be secreted by myoblasts of rectus femoris muscle at the fetal stage, but by fibroblasts in the muscle at neonatal and adult stages.