In a lecture in January 1896 WILHELM CONRAD ROENTGEN demonstrated a new kind of radiation (Xrays), which penetrated human tissue and blackened (or darkened) photographic films. In an early experiment, the X-rays had passed through the hand of his wife before impacting on a photographic film. On this film the shadow of the skeleton inside the fingers and the shadow of a metal ring around one of the fingers were whiter (brighter) and contrasted against the blacker (darker) shadows of the soft tissues of the fingers. The blood vessels in the soft tissues were invisible. Obviously, the skeleton and the metal ring, but not the vessels, attenuated the X-rays more than did the soft tissues.A few weeks later HASCHEK & LINDENTHAL (15) filled the blood vessels of an amputated hand with a suspension containing a skeletal component (chalk -a calcium compound) and a metallic component (Zinnober -a mercury compound). On roentgen films of the hand the vessels filled with the suspension were visible because they now absorbed X-rays more than did both the skeleton and the soft tissues. The first angiography had been made. The suspension had apparently functioned as a medium producing contrast in the roentgen films between the shadow of the vessels and the shadow of their surroundings. Since this beginning, there has been steady evolution towards the development of less toxic contrast medium compounds, allowing their use in vivo and making them important tools in diagnostic radiology.In this presentation an object means a structure of diagnostic interest in the body. We define a contrast medium as a substance which creates increased ob--ect contrast, i.e, an increased difference in attenuai;,;.,, ' . of X-rays between an object and its surroundi ;' 0~; ' This task can be performed only on the condi- tion that the contrast medium at some moment achieves different concentrations in the object and in its surroundings.The theme of this paper is a recently developed contrast medium, Visipaque (Nycomed). It is presently available for intravascular use as plasma-isotonic aqueous solutions of the nonionic (neutral) iodine compound iodixanol. These water-based solutions are enriched with sodium and calcium salts.I will try to give Visipaque a place in the ongoing evolution towards the never achievable goal -development of the ideal water-soluble contrast medium for angiography and for the extracellular space.The ideal contrast mediumIn the ideal world a contrast medium should have maximal efficacy (maximal attenuation of X-rays) and maximal safety (produce no adverse effects).In the real world maximal efficacy means that the medium should produce maximal object contrast in the human body, so that the X-ray detectors (roentgen films, intensifying screens, detectors of CT machines, etc.) can (from the X-rays coming out from the body) measure a difference in attenuation between that object and its surroundings and thereby produce a final image for the human eye with maximal image contrast between the images of the object and its surroundings.