1. The present investigation was undertaken to study the possibility that hypertensive vascular disease could be associated with changes in the morphology of the thymus.

2. Using light- and electron-microscopic techniques, the thymus of spontaneously hypertensive rats (SHR) with (1 year of age) and without (10 weeks of age) severe vascular lesions were compared with appropriate controls.

3. The thymus of SHR with severe hypertensive vascular disease showed: (a) marked hyperplasia and hypertrophy of the epithelial cells, with ultrastructural features of hypersecretory activity, and (b) in contrast to the paucity of plasma cells in the thymus of the control animals, the SHR had numerous plasma cells.

4. These observations may suggest the involvement of an autoimmunological mechanism in the development of the vascular lesions in SHR.

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