A histochemical and electron-microscopic study has been done on the parafollicular (C) cells of the thyroid in the gerbil. Histochemical tests revealed the characteristic presence of α-glycerophosphoric dehydrogenase and the presence of phenolic groups and indoleamines. Acid phosphatase levels were similar to those observed in the apical cytoplasm of the follicular cells. Electron microscopy revealed characteristic presence of noncisternal rough endoplasmic reticulum, well-developed Golgi apparatus, numerous prosecretory granules and a few electron-dense mature secretory granules. Most characteristic of these cells was the presence of numerous large, electron-lucent vesicles which often were packed in the cytoplasmic processes that were located subjacent to the follicular cells and close to the blood capillaries. Significance of these histochemical and ultrastructural findings is emphasized particularly in regard to the presence of monoamines and their possible role in the release of calcitonin and thyroxin