Dodds and Noble reported that a single massive subcutaneous dose of pituitrin in rabbits resulted in a severe anemia on the 4th to 5th day after injection. This anemia was macrocytic, hyperchromic and accompanied by a marked leucocytosis, reticulocytosis, and an increase in bile. Dodds and Noble are inclined to entertain the possibility “that the control of blood destruction by the reticuloendothelial system may be vested outside the system and may reside in the posterior lobe of the pituitary gland.” In view, however, of the well-known antidiuretic action of pituitrin, and the high water content of a rabbit's normal diet, it was thought that this anemia might well be due to serum osmotic changes resulting from water retention, rather than to a “hemoclastic principle” of the posterior pituitary. In over 20 preliminary experiments on 15 healthy adult rabbits, pituitrin was injected subcutaneously in the dose reported by Dodds, 200 units/kg. of body weight (10 cc. pituitrin per kilo). The animals were kept in individual metabolism cages on a diet of cabbage, carrots and lettuce ad libitum. At frequent intervals the serum osmotic pressure values were determined by the vapor pressure method of Hill, and serum specific gravity alterations were followed by the falling drop method of Barbour and Hamilton. Complete blood counts were frequently taken, and the reticulocyte, hematocrit, fragility to hypotonic saline solutions, icteric index and mean cell diameter values were closely followed. Urines were tested for hemoglobin and urobilinogen, and stools for occult blood. Body weight, urine volumes and specific gravities were recorded daily.