Mast cells have importance not only in their familiar role in acute allergic and parasitic diseases but also in chronic inflammatory, immunologic and fibrotic states. In addition they effective on thyroid gland function and the regulation of thyroid blood flow and capillary permeability. In this experimental study, hypothyroidism was induced by 10 mg/kg 6-n-propyl-2-thiouracil (Sigma), dissolved in 0.5 ml of 0.9 M saline, injected to 5-6 weeks old female rats intraperitoneally. Rats were sacrified after 15 days and thyroid glands were taken for light and electron microscopic studies. In the experimental groups, we observed changes in the follicular structure, along with decreased colloid , increased mitotic activity and intrathyroidal mast cells. At the ultrastructural level dilated GER was observed in the epithelial cell cytoplasm with on and increase in the intercellular spaces. Apoptotic cells were not observed in the follicular epithelial cells.