A precordial scanning technic and its application in the detection of left-to-right cardiac shunts in 75 patients are described. I131-labeled Diodrast was injected into an antecubital vein, its concentration in the central circulation was metered by means of a scintillation detector, and the time-concentration curve were recorded directly. In patients with left-to-right shunts these curves were modified in a characteristic fashion; the build-up time was normal, but the disappearance of isotope from the central circulation was slow. The product of the build-up time and disappearance rate was generally lower than in patients without cardiac shunts. The clinical value of this simple technic in detecting or excluding the presence of left-to-right shunts is discussed.