When metrizamide was introduced by Almén and later dimerized media were synthesized, they offered the possibilities of studying the effects of contrast media with equal iodine content but different osmolalities. This article will describe findings regarding the cardiac effects of the nonionic medium, metrizamide, and of a monoacid dimer (P-286) in comparison to conventional ionic media. In assessing the toxicity of contrast media on the heart, it has been useful to separately evaluate the effects on the mechanical functions of the heart and on the electrical processes of the heart. Other parameters, such as the effects on coronary blood flow and effects on enzymatic processes, may be assessed, but so far they seem less interesting in terms of describing toxicity. We have excellent physiologic methods to measure the effects of contrast media on the heart's mechanical functions, but the new contrast agents have negligible effects on these functions. The side effects of these new media on the electrical processes of the heart are probably less negligible, but our methods of assessing them are less satisfactory. Change in heart rate and various ECG changes can be recorded, but their correlation to the arrhythmogenic activity is uncertain. Recently, preliminary results have been reported concerning the effects of contrast media on the fibrillation threshold. This index may be a new, important approach of finding ways to minimize cardiac side effects of contrast materials.