Pain caused by x-ray contrast media in peripheral arteriography was assessed by behavioral changes of nonanesthetized, unrestrained rats. All ionic monomeric contrast media caused severe pain in a concentration of 300 mg I/ml. Dilution of contrast media markedly reduced pain. Sodium salt solutions were considerably more painful than meglumine salt solutions. The intravenous cholegraphic agent iodipamide did not cause vascular pain in a concentration of 300 mg I/ml, although its systemic toxicity is high. Pain in arteriography was attributable primarily to the high osmotic pressure of contrast media solutions rather than to their chemotoxicity.