Extracellular MRI and X-ray contrast agents are characterized by their phar- cokinetic behaviour. After intravascular injection their plasma-level time curve is characeterized by two phases. The agents are rapidly distributed between plasma and interstitial spaces followed by renal elimination with a terminal half-live of approximatly 1¿2 hours. They are excreted via the kidneys in unchanged form by glomerular filtration. Extracellular water- soluble contrast agents to be applied for X-ray imaging were introduced into clinical practice in 1923. Since that time they have proved to be most valuable tools in diagnostics. They contain iodine as the element of choice with a sufficiently high atomic weight difference to