Three different types of contrast media, monomeric ionic, dimeric ionic and monomeric non-ionic, were tested and compared concerning their toxicity on three experimental models: I. Pulmonary artery and aortic pressure after contrast medium injection into the right atrium in rabbits: II. Ventricular contractile force and heart rate after perfusion of the coronary arteries on the isolated rabbit heart: III. Aortic endothelium of rats. Non-ionic contrast medium was, in all three experiments, less toxic than monomeric ionic; thus it caused less rise in pulmonary artery pressure (I) and caused less reduction in ventricular contractile force and heart rate (II) and caused less damage on aortic endothelium of rats (III). The non-ionic contrast medium also gave less toxic reaction in two (I and II) of the three models compared to the dimericionic contrast medium. In experimental model III there was no difference between these two agents. Also, the dimeric ionic contrast medium caused less toxic reactions in all three models than the monomeric ionic contrast medium.