The non-ionic contrast medium metrizamide (Amipaque) was compared in animal experiments with diatrizoate, iothalamate and metrizoate. Vasodilatory effects, acute intravenous toxicity and effects on red blood cells, pulmonary arterial pressure, endothelium and heart were all less after metrizamide than after the ionic media. This motivated clinical trial of metrizamide for angiography, which demonstrated that metrizamide produced less pain than the ionic monomeric media in femoral as well as in other types of angiography.