Iomeprol for injection is a new nonionic triiodinated contrast medium for diagnostic radiology, which combines low osmolality with low viscosity. The effects of iomeprol for injection on the cardiovascular system, blood parameters, renal function and the central nervous system were studied after intravascular administration to several animal species of doses at least as high as the highest presumed clinical doses. The following observations were made with respect to the central circulatory system: moderate and short-lasting increases of left ventricular end-diastolic pressure and cardiac output, no significant effects on heart rate either in vitro or in vivo, some episodes of arrhythmia and ventricular fibrillations only at doses far higher than the highest presumed clinical ones, no significant increases in diastolic and systolic coronary flow. The following observations were made with respect to peripheral circulation: no significant changes on blood pressure, moderate and short-lasting increases in renal and pulmonary arterial flow, together with a decrease in peripheral vascular resistance, no crossing of the blood-brain barrier in healthy animals. Cardiovascular and haemodynamic changes were all significantly milder than those induced by ionic contrast media (CM) and were similar to effects caused by some other nonionic contrast media. When injected into the femoral artery of rats, iomeprol was shown to be less algogenic than iopamidol and iohexol. In comparison with the same reference CM, iomeprol affected to a lesser extent the filterability of red blood cells in vitro and showed a less marked effect on their deformability. When administered intravenously at very high dosages, iomeprol had no effect on the glomerular filtration rate, but increased both renal blood flow and diuresis. Proteinuria and enzymuria were also increased, albeit more transiently. The neurotolerance of iomeprol for injection after intravenous administration was higher than or at worst equal to that of iopamidol and iohexol. Iomeprol is therefore a promising new contrast agent particularly suitable for intravascular use in humans.