Moderate and severe levels of experimental valvular aortic stenosis (VAS) were produced in anesthetized, open-chest dogs to determine the effects of VAS on subepicardial (EPI) and subendo-cardial (ENDO) blood flow, O2extraction, and O2consumption (MVO2). Regional flow was determined by microspheres, and O2saturation and extraction were analyzed by a three-wavelength absorbance microspectrophotometric method. Left ventricular pressure-volume work increased by 49% in moderate and 135% in severe VAS. The ENDO:EPI flow ratio averaged 1.21 ± 0.09 in controls and decreased to 0.90 ± 0.16 in moderate and 0.89 ± 0.09 in severe VAS, and coronary flow increased by 40% and 58%, respectively. O2 extraction increased with both moderate and severe VAS, with extraction being higher in the ENDO than the EPI. MVO2increased more in severe VAS than in moderate VAS. The ENDO: EPI MVO2 ratios fell to 1.10 (moderate) and to 0.84 (severe) from 1.44 in controls. We conclude that the O2supply and consequently the MVO2became limited in the ENDO relative to that in the EPI by simultaneous limitation of the O2extraction reserve and the blood flow to this region during the increased O2requirements imposed by experimentally induced VAS. Circ Res 46: 139-145, 1980