(1) A population of cells exists in the thyroid gland (of the dog and other mammals) which is entirely distinct from the population of follicular or acinar cells. (2) The cells were originally described in 1877 by Baber, in the dog thyroid, and later by Hurthle (1894) and by Nonidez (1931). They are characterized cytochemically by a content of basic protein or polypeptide, by argyrophilia, and by high $\alpha $-glycerophosphate dehydrogenase. At the electron microscope level, their cytoplasm is seen to be filled with spherical vesicles 1500 to 2000 degrees A in diameter, bounded by a single membrane, and their mitochondria are especially electron dense. (3) These cells, the C cells, respond to high blood calcium levels by increased secretion of the product contained in the spherical vesicles. (4) It is postulated that this product is thyrocalcitonin, the calcium regulating hormone of the thyroid gland. If this premise is correct the C cells constitute a new endocrine system.