Injury to the supra-opticohypophysial system in dogs with hypertension of the Goldblatt type was associated with statistically significant declines in mean arterial pressure in 5 of 8 cases. There was also a fall in blood pressure in one dog with a spontaneous hypertension presumably of renal origin. Normal levels were not usually reached. No correlation was observed between the degree of reduction in blood pressure and the extent of neurohypophysial dysfunction as indicated by intensity of the polyuria. Pitressin in oil did not re-elevate the blood pressure in 1 case. Reasons for and against regarding the alterations in blood pressure as due to the lesions of the neurohypophysial system as well as for and against regarding them as due to ant. lobe injury are presented. It appears possible that denervation of the hypophysis by the methods used, known to affect particularly the pars neuralis, may so disturb the internal environment in some animals that conditions for the complete maintenance of a renal hypertension are altered and a decline in mean arterial pressure results.