Vacuolar degeneration of the thyroid follicular epithelium was observed in two untreated female cynomolgus monkeys assigned to control groups. In light microscopy, large vacuoles containing a homogenous substance occupied the basal region of the epithelium, and the nuclei had shifted toward the apical region. The vacuoles showed negative reactions to PAS and thyroglobulin. Electron microscopic observation revealed dilatation of the rough endoplasmic reticulum corresponding to the vacuoles. The plasma TSH, T3 and T4 levels determined for the samples kept frozen were within the normal ranges, suggesting that the thyroid function was kept intact.