Hypertension has been shown to increase the severity of atherosclerosis in both man and experimental animals (1). Most observers attribute this effect of hypertension to local damage to the vessels produced by the augmented pressure or turbulence of the blood. This concept is supported by the more frequent occurrence and greater severity of atherosclerotic lesions at locations of high pressure or turbulence. It has also been suggested that high blood pressure increases the filtration of blood across arterial walls and, presumably, affords an opportunity for retention of lipid in the walls (2).The present report is concerned with two other factors that may affect the concentration of lipid in the aortas of hypertensive rats: the concentration of lipid in the blood, and lipid synthesis in the aorta itself. Cholesterol concentrations were found to be significantly higher in aortas of hypertensive rats than in those of normotensive controls. This difference was observed when hypertensive animals had higher serum cholesterol levels than their controls, but the difference persisted when experimental conditions were adjusted so that the serum cholesterol levels were comparable in the two groups of animals. It was found that the rate of cholesterol synthesis in vitro is increased in the aortas of hypertensive rats. A Gaertner cuff 22 mm long 'and an "Infraton" 4 pickup were used.Parabiotic pairs. Wistar rats were inbred in this laboratory by mating siblings for at least ten generations; CFN rats were inbred in the same way for three generations. Parabiotic pairs were prepared by suturing the previously divided lateral abdominal musculature of the two rats while maintaining separation of the peritoneal cavities and joining the scapulae.Cholesterol concentration in serum. To obtain blood for the measurement of serum cholesterol levels at intervals, rats under light ether anesthesia were bled by