Iohexol-d5 is intended for use as an internal standard for the quantification of iohexol by GC- or LC-MS. Iohexol is a non-ionic and water-soluble contrast agent.1 It induces inhibition of electrical activity but has no excitatory effect in vitro in rat hippocampal slices and lacks any effect in vivo on ventral root flexes of feline spinal cord, indicating minimal neurotoxicity. Iohexol also induces a smaller decrease in contractile force than the ionic and high osmolarity contrast agent metrizoate in isolated rabbit hearts perfused with a 350 mg/ml solution.2 Formulations containing iohexol have been used for visualization of arteries, veins, the urinary tract, and joints using X-ray and computed tomography (CT) scan imaging techniques.