A safe and reproducible technique to create supravalvular aortic stenosis was developed, which avoids many of the difficulties encountered in the production of aortic stenosis. Dogs were anaesthetised and artificially ventilated. The chest was opened and the venae cavae were encircled with umbilical tapes. The ascending aorta was then encircled by a 1.5-2 cm wide, 6-7 cm long dacron patch, venous return was stopped by tightening the tapes, and a J-shaped clamp applied to the ascending aorta at the dacron patch. Two layers of continuous mattress suture were placed adjacent to the clamp, plicating the aortic diameter by about 50%. After releasing the clamp and restoring normal venous return, left ventricular (LV) and aortic (AO) pressures were measured. Subsequently, one or two deep mattress sutures were placed below the running mattress sutures to increase the stenosis and to obtain the desired gradient. The LV-AO systolic pressure gradients obtained immediately after the operation ranged from 40 to 75 mm Hg. Two to 6 months after the operation the pressure gradients ranged from 50 to 200 mm Hg. Left ventricular to body weight ratios were 6.41 (SEM 0.26) v 4.24(0.20) for the controls. Heart weight to body weight ratios were 8.37(0.35) v 5.65(0.33). LV end diastolic pressures were normal. This technique can be used either in puppies or adult animals. The problem of aortic rupture is eliminated. The pressure gradient can be easily controlled during the operation and reproducible LV hypertrophy can be obtained in a shorter time than with aortic banding of puppies.