Thoracic aortic histamine synthesis has been examined in rats rendered hypertensive for periods ranging from ¼ to 8 days through ligation of the abdominal aorta between origins of the renal arteries, in a preliminary attempt to establish whether activity of the aortic nonmast-cell histidine decarboxylase system is increased in experimental hypertension. Results indicate that aortic histamine synthesis is increased 66, 75, 96 and 55% at postsurgical intervals of ¼, ½, 1 and 8 days. These data suggest at least one biochemical explanation for both increased arterial wall permeability and the arterial inflammation consistently associated with both clinical and experimental hypertension by resolving these into those associated with the prolonged inflammatory respon