Radioactive contrast media, such as I131-labeled Diodrast, Miokon, or Hippuran, have been widely used for renal function studies. They are entirely unsatisfactory, however, for renal scintillation scanning because they are rapidly excreted by the kidneys, without retention in the renal parenchyma (Fig. 1, A). In contrast, mercurial diuretics labeled with radioactive mercury are concentrated to a high degree in the renal tubular cells (2), chiefly in the cortex, prior to their appearance in the urine (Fig. 1, B). Furthermore, the nuclide Hg203 has a single, rather weak gamma emission (280 kev) that enables good resolution when a multihole focusing collimator and gamma spectrometry are used (1). These pharmacological and physical characteristics make it possible to obtain a clearly delineated spatial image of the kidneys in man by means of scintillation scanning. The mercurial diuretic is retained in the renal parenchyma long enough to permit completion of the procedure. The effective half-life of the com pound within the body is nevertheless quite short (average, three hours), so that the radiation dosage is minimal (less than 0.5 rad), despite the long physical half-life of Hg203 of forty-five days. Following an intravenous dose of 100 to 150 microcuries of Hg203 Neohydrin, maximum renal concentration usually occurs within one to two hours. The scanning procedure is performed with the patient prone, to minimize the detector-tokidney distance. In a preliminary trial of this procedure in 25 patients, a good image of the renal parenchyma has always been obtained in the absence of uremia. In several instances of mild uremia (serum urea nitrogen 40 to 80 mg./100 c.c.) the size and shape of the kidneys have been demonstrated when intravenous pye-lography has shown no visualization. When the uremia has been more severe, the labeled diuretic concentrates in the liver as renal concentration becomes poorer. Space-occupying lesions within the kidney, both cysts and tumors, have been demonstrated as filling defects (Fig. 2). The method has proved valuable in differentiating fetal lobulations distorting the renal margins from true intrarenal masses. Polycystic renal disease has produced a picture not only of generalized kidney enlargement, but the cysts have been apparent as multiple filling defects. A great potential of this method may be the demonstration of localized areas of renal ischemia. In two patients, ischemic renal segments have been revealed which were not clearly discernible by pyelography. With anticipated improvements in resolution, it is hoped that space-occupying lesions or ischemic areas as small as 1.5 cm. in diameter may be demonstrated within the kidneys by the scanning technic. Resolution may be further improved by using a nuclide with an even weaker gamma emission. Co57-labeled Versenate concentrates in the renal parenchyma and promises to be satisfactory for this purpose.