Electrocardiographic monitoring of 406 patients undergoing intravenous urography was performed before, during, and after the examination. Major cardiac arrhythmias and ischemia were encountered frequently (18%) in those with cardiac disease but also occurred (5%) in the healthy individual with no history of heart disease. Rapid higher dose (28-g iodine) bolus injections result in more cardiac alterations than the slower but larger (42-g iodine) infusion method, whereas the smaller (14-g iodine) bolus injections have the least cardiac effect. Ectopic ventricular beats, the most common abnormality, are usually transient but remain the most potentially lethal of the effects of intravenous contrast media on the heart.