Purpose: To investigate whether iodinated radiographic contrast media (IRCM) mimic the hyperpolarizing and vasodilator effects of K + by comparing the vasodilator effect of a transient rise in extracellular K + with that of the IRCM iohexol. Material and Methods: Immersed rabbit central ear arterial rings with and without endothelium and pre-contracted with phenylephrine (PE) were used to investigate the dependency of the endothelium in K + -induced vasodilatation. Perfused rabbit central ear arteries, pre-contracted with PE, were used to study the effects of bolus administrations of the IRCM iohexol or KCl on arterial tone under conditions that mimic those employed during clinical arteriography. Results: A small rise in K + caused an endothelium-independent and ouabain-sensitive relaxation of PE-constricted rabbit central ear artery rings. The relaxation was not changed in the presence of barium. The IRCM iohexol and KCl, injected as boluses into perfused PE-constricted rabbit ear arteries, caused transient decreases in perfusion pressure. Iohexol- and K + -induced pressure decreases were significantly reduced in the presence of 10M ouabain alone or in combination with 30M barium. Neither iohexol- nor K + -induced pressure decrease was significantly changed in the presence of barium alone compared to controls. Conclusion: The vasodilator effect of IRCM mimics the vasodilator effect seen upon small increase in extracellular K + . Under the experimental conditions employed in the present study, a considerable part of the IRCM-induced vasodilatation appears to be due to activation of Na + /K + -ATPase in the smooth muscle cells. © 2006 Taylor and Francis.