The permeability of the skin of fetal rats between 14 and 21 days of intrauterine life was investigated by injecting lanthanum nitrate solution subepidermally and by exposing the external surface of other fetuses to the same solution. Up to the age of 18 days, the epidermis was freely permeable from the basal surface and lanthanum entered junctional complexes between periderm cells. After 18 days, the limit of permeation corresponded to the level at which membrane-coating granules were released into the intercellular spaces. At all ages, permeation from the outer surface was limited to the junctional complexes between the periderm cells covering the epidermis throughout fetal life. Damage to the periderm at early ages resulted in complete penetration of the epidermis by lanthanum. It was concluded that, prior to secretion of the epidermal permeability barrier at 18 days, the periderm was the principal barrier to permeation by lanthanum.