Fibronectin is a well known glycoprotein of extracellular connective tissue matrices due to a specific amino acid-sequence (RGD) suggested to act as an attachment factor in cell-cell or cell-matrix interactions. Although also present in dermis of skin, little is known about the distribution of fibronectin in the skin. To obtain data for distribution on skin, we used immunolocalization techniques to quantitatively examine the distribution of fihronectin in various layers of skin by developmental stages of rats. The study was focused on fibronectin distributions of skin in 7 developmental stages of newborn rats. The results were obtained as follows. 1. Stratum lucidum and granulosum in epidermis showed the strong fibronectin immunoreactivities from day 1 to day 5 neonatal stages of rats. But at the 1st week after birth, fibronectin immunoactivities on the epidermis were diminished as much as those of control group rats. 2. At 1 day and 5 day after birth, strong immunoreactivities of fibronectin were seen in papillary layer of dermis moderate or wreak immunoreactivities in reticulum layer of dermis. 3. At the epidrmal-dermal bored(basement membrane) and hair follicles, moderate fibronectin immunotivities were increased from 1 day to 5 day after birth. But from the 2nd week after birth, immunoreactivities of fibronectin were decreased to be weak reactions. It is suggested that large amounts of the fibronectin distribution in the neonatal rat skin will be devoted to rapid formation of dermal tissue.