A study of the changes in the Hb, erythrocytes, sp. gr., and water content of the blood was made in 19 dogs during a period of de hydration and recovery. Thirteen of the exptl. animals were dehydrated, without apparent discomfort, by inanition and water deprivation for 7-10 days. This chronic de hydration was accompanied by an increase of Hb from 15.9 g. 207 g. /100 ml. of blood in 7 days. The red erythrocyte count increased from 5,150,000-6,702,000 cmm. and the sp. gr. changed from 1.04528-1.05201. The water content of the blood decreased 1%. Dehydration for longer than 7 days was followed by a decline in the blood constituents to, or slightly below, the normal level. Upon return to the normal diet, there was a sudden hemodilution followed by a gradual return to the normal condition in 25 days. Acute dehydration under pentobarbital anesthesia produced by hemorrhage of 9-20 g. of blood/Kg, of body wt. in 4 dogs showed characteristic hemodilution. Acute dehydration by the injn. of 50% sucrose in 2 dogs was followed by hemoconcn. Hb increased from 14.5 g.-16.5 g./100 ml. of blood. The erythrocyte count increased from 4,590,000-5,210,000 cmm. and the sp. gr. was changed from 1.04560-1.05140. Acute dehydration following a period of chronic dehydration showed little effect upon the blood picture.