The publication of several recent reports suggesting the use of zirconium for contrast radiography (3, 6) prompts us to publish our experience with this element. Zirconium is element number 40, with an atomic weight of 91.2. The metallic element does not occur free in nature, but is commonly obtained as the silicate, oxide, etc. Zirconium minerals, chiefly zircon (ZrSiO4), are found in alluvial deposits derived from igneous rocks. Zircon is a heavy crystalline material with low solubility and remarkable refractory properties. It has been used industrially as a substitute for silica and sand. Zirconium is usually quadrivalent, although occasionally it is bivalent, as in zirconium hydride (ZrH2). Hydrated zirconium oxide is amphoteric in that it acts both as an acid and as a base. In general, there are three principal types of zirconium salts. 1. The normal salts in which the zirconium acts as a quadrivalent cation in combination with the radicles of the stronger acids, e.g., Zr(SO4)2. 2. A series of basic salts in which the bivalent zirconyl radicle (ZrO) acts as a base. These salts can often be considered as intermediates in the progressive hydrolysis of the normal salts. 3. A series of zirconates or metazirconates in which the zirconyl hydroxide (Zr(OH)2) acts as an acid (metazirconic acid (H2ZrO3) ). There is a tendency for the acid to form polyzirconates analogous to the polysilicates. The use of zirconium compounds for contrast radiography is not new. As early as 1909, Kaestle (4) advocated insoluble zirconium oxide (ZrO2) as a contrast medium for the gastrointestinal tract. In 1948, McClinton and Schubert (6) briefly alluded to this substance for the same purpose. Hunter and his coworkers (3) in 1949 reported their experience with sodium zirconyl citrate as a radiopaque medium and commented upon the promising future of such compounds. A review of the chemistry and biology of zirconium suggests that this element might be an ideal contrast medium, since it is relatively non-toxic and has none of the disadvantages of the organic halogen derivatives. Kaestle found no toxicity following the subcutaneous injection of 400 mg. of a soluble zirconium salt into rabbits, and Harding (2) injected a finely divided 10 per cent suspension of zirconium silicate intravenously, intraperitoneally, and intratracheally without toxic effect. Schubert and his associates (6–10) confirmed the low toxicity of various soluble zirconium salts in rats. These investigators observed no toxic effects following injection of as much as 210 mg./kg. of zirconium citrate intravenously in a single dose and 1,336 mg./kg. in multiple doses. Cochran et al. (1) administered a number of zirconium salts to rats (sodium zirconyl sulfate, zirconyl acetate, nitrate, sulfate and chloride) and also confirmed their low toxicity.