Scanning electron microscope observations were conducted on cross-sections and bloodvessel casts of normal rat thymus. A comparative study was carried out with thymus crosssections prepared from rats injected with corticosteroid and from rats undergoing muscleembedding transplantation. The results were as follows: Cortical lymphocytes of normal rat thymus had fewer microvilli, while medullary lymphocytes had many. The network made by epithelial cells was smaller but that made by the medullary cells was larger. The cortex medulla boundary was distinct on the cortical side and indistinct on the medullary side. The interior surface structures of the posterior capillary venules presented a variety of changes which enabled smooth wandering of the cells. There were also some lymphocytes penetrating through the vascular endothelium. With blood vessel casts cicatrization of the posterior capillary venules was characteristic. On the interior surface of the thymus cyst there were villous or cribriform structures. In the cortex of the rat thymus injected with corticosteroid destruction of lymphocytes was marked, and in the medulla a decrease in the number of lymphocytes was the main change. In the rat thymus after muscle-embedding transplantation cortical lymphocytes appeared rounded and there were many ciliated cells.