Selective vertebral angiography was performed in 29 rabbits in order to compare the adverse effects of three monomeric (iopamidol, iopromide, metrizamide) and one dimeric (iodecol) non-ionic water-soluble contrast medium. The dose was 10 ml and the iodine concentration 350 mg/ml. Marked, but transitory changes were seen in the electrocardiographic and blood pressure recordings with all the contrast media. The monomers seemed to have a higher tendency than the dimer to cause spasm in the intracranial arteries, while the changes in the cardiovascular recordings were more marked with the dimer. One animal died and one animal developed a permanent neurological deficit following the injection of the dimer. All the other animals recovered completely and seemed quite unaffected within half an hour after completion of the experiment.