The purpose of this study was to investigate the effect of hydrocortisone in the fusion mech ani in of the secondary palatine shelves in the tat. Pregnant rats were injected with 5.0 mg/100 gm body weight of hydrocortisone for 3 days between the 7th day and the 9th day of pregnancy. Between the 14th day and the 18 day of pregnancy, the fetuses´ were removed and decapitated to be immersed in 10% formalin ,and-Carnoy´s solution. Preparations were stained with alizarin red S, hematoxylin-eosin and alcian blue respectively, and partly were treated for Periodic acid-Schiff reaction. The results were as followings: 1. The incomplete fusion of the palatal epithelium took place though the polarity of epithelial cells. 2. At the edge of shelves the differentiation of mesenchymal cells was observed, but the interpenetration of mesenchyme was not shown. 3. It was considered that the phenomenon of hypocalcification in matrix had relation to the decrease stainability to alizarin red S in shelves. 4. It might be concluded that the connective tissue under epithelium showed the decrease tendency to alcian blue and PAS reaction due to the inhibition of the synthesis of the mucopolysaccharide.