The most important recent investigations of the cytology of the parathyroid gland in mammals are those of Rosof [1934] and de Robertis [1940,1941], both of whom studied the rat. These investigators described the Golgi apparatus and mitochondria, the former correlating changes in them with the phases of a secretory cycle and the latter describing a reduction of the Golgi material after injection with parathyroid extract and a hypertrophy after the experimental induction of rickets on a low-calcium and low-phosphorus diet. Both workers observed dark osmiophil and light non-osmiophil cells, and de Robertis described a disappearance of the dark cells following parathyroid-extract injection and an increase with low-calcium and low-phosphorus diets. With a view to carrying out similar experimental studies on the mouse parathyroid the present investigation of the normal cytology was first undertaken. It was also of interest to discover whether the rather complicated secretory cycle described by Rosof was