Hydropic change of follicular epithelial cells in the thyroid was observed in a female Fischer 344 rat. Microscopically follicular epithelial cells were characterized by edematous swelling with weakly eosinophilic and homogeneous cytoplasm. The cytoplasm was negative for periodic acid-Schiff reaction, and thyroxine- and thyroglobulin-immunohistochemical reactions. Electron-microscopically, a small amount of amorphous substance was noted in remarkably dilated rough-surfaced endoplasmic reticulum (r-ER), and slight regressive changes of cytoplasmic organella were also observed. These morphological changes may indicate that focal intracytoplasmic edema was occurred in r-ER, and that the change belonged to hydropic degeneration of the thyroid follicular cells in the thyroid.