Cardiac output and ejection fractions of the cardiac ventricles are valuable measures of cardiac function. Noninvasive radionuclide techniques for determination of these measures are based on analysis of different data: first passage low frequency, first passage high frequency and multigated equilibrium data. We have evaluated the principles of these methods using a dynamic cardiac phantom. Radiocardiographic determinations of cardiac output and right and left ventricular ejection fractions showed excellent correlation against true phantom values: r = 0.993 (p less than 0.001), r = 0.978 (p less than 0.01) and r = 0.943 (p less than 0.05), respectively. Thus, the findings of the present study demonstrate a high validity of current radiocardiographic measures of cardiac function.