In spite of extensive efforts by several investigators in the late 1950's to perfect "in transit" scanning of continuously excreted contrast agents labeled with radioactive iodine (131I), renal scanning did not become a popular procedure until the introduction of chlormerodrin Hg 203. With this material, approximately 60% of the injected dose is in the kidneys within one to two hours. Although the agent is eventually excreted so that only about 10% ultimately remains fixed in the renal parenchyma, the excretion is slow. As a result, at one hour, renal scanning can be done with assurance that the renal isotope concentration will remain reasonably constant throughout the scan. Since all the renal scanning agents currently in use localize in the renal parenchyma (principally in the cortex), the scan has been used to (1) identify the degree of parenchymal function—often possible when the kidneys are not visualized on intravenous pyelograms; (2)