A simple technique is described for producing severe reproducible renal hypertension in the rat. Total ligation of the aorta between the renal arteries and just below the origin of the superior mesenteric artery resulted in sustained systolic and diastolic pressures in 90 per cent of 170 rats studied. Arterial pressure is then measured in the conscious unrestrained rat through a carotid cannula inserted no more than 48 hours before measurements are made. The mean arterial pressure increases to a peak of 180 mm. Hg at day 5 and then remains at a plateau of 160 mm. Hg through the 40 days of the study. Plasma renin increases to a peak at 5 days but returns to baseline at 30 days despite the persistence of severe hypertension. Infarction of the left kidney below the aortic constriction results in no increase in pressure or plasma renin. Infarctions of the right kidney by emboli originating from the indwelling carotid cannula are associated with greater increases in blood pressure and plasma renin than the standard preparation. Prolonged carotid cannulation must be avoided in order to prevent such emboli. This simple technique for producing renal hypertension allows studies on the pathogenesis of hypertension with an excellent degree of reproducibility and reliability.