Mesenteric arteries and saphenous veins of control and perinephritic hypertensive dogs were studied with light and electron microscopy. 2 weeks after surgery there are ultrastructural changes, both in arteries and veins which are thought to be of hydroelectrolytic origin. These alterations are more marked 7 weeks after surgery and disappear thereafter. There are signs of increased protein synthesis activity in the smooth muscle of the mesenteric arteries, but not of the saphenous veins obtained 7 and 14 weeks after surgery, leading to a progressive hypertrophy of the muscular cells. The connective tissue augmented proportionally. Degeneration of adrenergic nerves in the arteries, accompanied by a parallel decrease in their noradrenaline content, constituted an unexpected finding clearly deserving further study.