A method is described for measuring left ventricular ejection fraction which uses high frequency computer recording of gamma scintillation camera data and peripheral venous injectinon of technetium-99m as sodium pertechnetate. Data from mechanical model experiments are used to show feasibility of this method. A phantom experiment is described which was used to develop a technique for accurate delineation of the ventricular outline in the presence of background. The left ventricular ejection was measured in 12 patients by radionuclide angiocardiography and biplane cineangiography. Comparison of these two methods gave a correlation coefficient of 0-91. In addition, left ventricular ejection fraction was measured in 34 patients (aged 7 weeks to 18 years) without evidence of cardiac disease using the radionuclide method alone. Average ejection fractions of 0-66 and 0-70 were found for children over 2 years of age and children 2 years of age or younger, respectively. In addition, an interobseerver comparison study was performed with the data from 10 patients, and only small differences were noted (SD 0-025).