The effect of aortic stenosis on total, phasic, and regional myocardial flow was studied in 16 anesthetized, open-chested dogs. An adjustable catheter device was used to produce increasing aortic obstruction, the severity of which was judged from the left ventricular aortic peak systolic gradient as mild (< 25 mm. Hg), moderate (26 to 50 mm. Hg) and severe (> 50 mm. Hg). The supply/demand index was estimated from the ratio of diastolic pressure time index to systolic pressure time index ( DPTI SPTI ). The ratio of diastolic to systolic coronary blood flow ( DIAS SYS ) was determined from the flow tracings. Total myocardial flow and endocardial/epicardial flow ratios ( ENDO EPI ) were determined by injecting four differently-labeled 7 to 9 micron microspheres in the left atrium during control, mild, moderate, and severe aortic stenosis. The supply/demand ( DPTI SPTI ) index decreased significantly at all levels of aortic stenosis because of a decrease in DPTI and an increase in SPTI. The DIAS SYS coronary flow ratio and the ENDO EPI myocardial flow ratio were not significantly changed during aortic stenosis. The total myocardial flow was significantly higher than control only during severe aortic stenosis. The results indicate that, for all degrees of experimental aortic stenosis, there were significant decreases in DPTI SPTI but no significant changes in DIAS SYS coronary flow ratio or the distribution of myocardial perfusion. Thus, in acute, experimental aortic stenosis, there is evidence of increased myocardial oxygen demand, but endocardial perfusion is not changed significantly.