Meglumine iothalamate, an iodinated contrast agent, is commonly used in humans for roentgenographic outlining of cerebral ventricles. Although clinical studies have shown meglumine iothalamate to be less toxic than other radiopaque materials, its microscopic effects on brain tissue have never been studied in a controlled manner. For this study canine cerebral tissue was exposed to intraventricular saline or saline plus meglumine iothalamate and the results were compared by light microscopy. Reactive changes were found in both groups. They occurred with greater frequency and degree in the group exposed to meglumine iothalamate, but permanent damage to brain tissue, did not occur.