The purpose of this study was to evaluate possible vasoconstrictive or vasodilatator effects of the low osmolar non-ionic contrast medium iohexol (Omnipaque) on the calibre of cerebral arteries. The diameters of respectively 5 and 6 different locations of the vertebral and carotid arteries were recorded from angiograms of 3 successive injections. The material consisted of 18 patients. It could not be shown that Omnipaque induced vasoconstriction or vasodilatation when normal doses and adequate technique was applied. The difference in diameters found were not statistically significant and are probably due to natural pulsations and to the difficulties in performing exact measurements.