Measurement of disappearance rates of diffusible isotopes with a scintillation camera has been used to estimate myocardial blood flow in man, although there has been no data to assess the accuracy of the technique. We compared regional flow measurements using scintillation camera detection of 133 Xe with essentially simultaneous measurements using gamma-emitter labeled microspheres and differential spectrometry of left ventricular tissue. In 16 dogs, flows through 20--50% of the left ventricle were obtained at rest and with intravenous isoproterenol; the coefficient of correlation of paired measurements with the two techniques was 0.74. In eight dogs, a coronary artery was ligated, and regions approximately 2.2 cm2 at nearby locations in normal, infarcted and marginal tissue studied; the coefficient of correlation was 0.73. In five dogs, flow in three to five precisely identical 1.1 cm2 regions were measured at rest and with isoproterenol by both methods; correlations within each dog ranging from 0.81 to 0.93 were obtained. Although flows with the scintillation camera technique tended to be lower than values with the microsphere technique, it is suggested that the linear correlations of paired values with the two techniques is evidence that scintillation camera detection of 133Xe can be used to quantitate changes in regional myocardial perfusion in the cardiac catheterization laboratory.