Red blood cells labelled with radioactive Technetium were injected intravenously, and the passage of the tracer through the cardio-pulmonary system recorded using a scintillation camera coupled with a numerical dosimeter. The results are presented as serial analoque pictures taken at the rate of two per second, as numerical tracings which are later processed, and as flow charts from predetermined areas of interest: the chambers of the heart, the lung parenchyma, and the aorta. This study is concerned with 58 patients with a left-to-right shunt, and about 40 normal examinations were used as a baseline for comparison. In cases of left-to-right shunt, the finding on the tracings is of abnormally prolonged activity in the lungs after a normal passage through the right side. The intensity-duration curves highlight this finding, and show an early recirculation peak in the right side of the heart and a slowing of pulmonary emptying, which can be evaluated numerically from the ratio C2/C1. A correlation has been found between this value and the ratio of pulmonary to systemic flow as calculated by oximetry. The correlation is even closer if a comparison is made between the curves of pulmonary radioactivity and the dye dilution curves. In certain cases, the shunt can be localised to the atrial or ventricular level. There are many indications for this safe investigation: --the diagnosis and monitoring of the flow through a left-to-right shunt in a child; --confirmation of the closure of septal defects after surgery; --clarification of an infudibulo-pulmonary syndrome.