The cardiac output may be determined from a radiocardiographic tracing. This method is somewhat tedious because it requires, in addition to the tracing, the determination of the total blood volume. The usual method for calculating the cardiac output from the radiocardiogram is a modification of that of MacIntyre et al. (1951): This formula contains three variables and one constant. The three variables are blood volume, measured value obtained for the equilibrium of the distribution of the tracer in the blood, and the area under the radiocardiographic tracing. Heretofore, the blood volume has been determined independently from the radiocardiographic tracing. From the following equation, it can be seen that once the equilibrium and the area have been measured, it is possible to obtain a numerical ratio which exists between the cardiac output and the total blood volume: The above ratio represents an index of the speed of the circulation, since it shows what fraction of the blood volume must have been ejected...