Mechanical factors that can modify the peak transvalvar pressure differences (delta P) in aortic stenosis were evaluated in a model. A latex rubber sac simulated the ventricle. Expansion of the walls of the sac by means of a negative pressure applied to its outer wall introduced a measure volume into the sac and placed the wall materials under tension. The stretched sac was then permitted to contract and to expel its contents through "aortic valvar" orifices of various severities of stenosis, into an aortic standpipe of selected diameters (compliances). Factors that increased the peak delta P included the strength (thickness) of the ventricular wall, the rate at which it mobilized and applied its tensile force to compress the sac contents, the unstressed volume of the sac, the total volume in the sac at onset of contraction, the severity of the valvar stenosis, the compliance of the aorta, the rate of arterial run-off, and the aortic diastolic pressure. Loss of forward stroke volume due to mitral regurgitation lowered the peak delta P. Elevations in diastolic arterial pressure also lowered delta P. All of these mechanical factors should be considered in the analysis of the severity of clinical aortic valvar stenosis and in decisions for medical therapy and surgical correction. The several factors which do not depend directly on the orifice area or on the forward stroke volume vitiate the sole use of the orifice formula in the analysis of the dynamics of aortic stenosis. The application of this approach in related problems is indicated.