The etiology of essential hypertension is still obscure. Perhaps subsequent research will disclose multiple etiologies and will demonstrate the term to be generic rather than specific. Among the numerous postulates that have been advanced to explain the genesis and maintenance of the hypertension which is pathognomonic of this disease is that of the possible presence of a pressor substance in the blood. Thus Danzer, Brody and Miles1 found a rise in blood pressure after the intravenous injection of unchanged specimens of blood from patients with hypertension into atropinized cats that had been desensitized to human blood, and concluded that there is a pressor substance in the blood in hypertension. Using the same technic, Danzer2 later reported similar results from a majority, but not from all, of the tested specimens of blood of patients with hypertension. On the other hand, Stewart3 obtained no pressor effects from the intravenous