Regional myocardial perfusion was measured in 164 patients at coronary arteriography. Washout of xenon-133 from multiple areas of the heart was monitored with a multiple crystal scintoooillation camera after tracer injection int the left or right coronary artery. Rate constants of radioisotope clearance were computed by monoexponential analysis of the initial portion of each washout curve. Regional myocardial blood flow rates in ml/100 g.min were calculated by the Kety formula. The pattern of local myocardial perfusion rates was compared to the coronary arteriogram obtained during the same study. In patients with normal coronary arteriograms, the average mean myocardial perfusion rate in the left ventricle (LV) significantly exceeded that of the right ventricle and the right atrial area. Mild heterogeneity of local myocardial flow rates in the LV was observed. In patients with essential hypertension, aortic stenosis, and aortic insufficiency, the average LV perfusion rates were similar to those of subjects with normal coronary arteriograms, pressures, and ventriculograms. The mean LV perfusion rates were significantly reduced in patients without coronary disease who had cardiomyopathy and cardiac failure.