41 albino rats were injected intraperiton. with physiologic saline in amt. equal to 15 cc. per 100 g. body wt. After 24 hrs. 11 animals had an elevated blood pressure, and 30 did not. Of the hypertensive animals, 4 had evidence of blood dilution based upon change of refractive index of plasma, and the remaining 7 had an increased blood vol. Of the animals with normal blood pressure, 18 had no evidence of dilution or of increased blood vol.; 8 had evidence of some dilution or increase in blood vol. but less than was present in the hypertensive animals; 4 showed evidence of dilution or of increased blood vol. to a considerable degree. The 4 in this last group were all young animals. In no case did a vascular hypertension occur without evidence of plasma dilution or of increased blood vol. Under the conditions of the expt., blood pressure and blood vol. appeared to vary directly.