The most important complications of intravascular administration of contrast agents include idiosyncratic (anaphylactoid) reactions, shock, congestive heart failure, cardiac arrhythmias, acute renal failure, and neurotoxic effects. The incidence of serious neurotoxic effects is low. Entry of contrast agents into the central nervous system normally is limited but may be increased by osmotic opening of the blood-brain barrier with cerebral arteriography or arch aortography. Most neurotoxic effects are thought to represent direct effects of the contrast agent on brain or spinal cord. Adverse effects with arteriography include seizures, transient cortical blindness, brain edema, and spinal cord injury. Most cases of focal brain deficit (other than cortical blindness) are attributed to embolism secondary to the catheter. Seizures may occur with intravenous administration, especially in patients with brain tumors or other processes disrupting the blood-brain barrier. The most important adverse effects observed with myelographic agents include acute and chronic meningeal reactions with iophendylate, and seizures and transient encephalopathy with metrizamide.