To observe the effect of cortison on the oxytalan fibers of the periodontal membrane, the albino rats of each groups were injected intramuscularly with 2.5mg. of cortison/kg. body weight daily for 3,7,10,14, and 21 days. After 14 days of cortison administration, the distributions of the oxytalan fibers were irrgular. In their path from cementum to alveolar bone, the fibers pass wave-like. Their density was loose and their stain ability was lesser than the normal. Prolonged administration of cortison impaired the development and the normal form of the oxytalan fibers.