Experiments were conducted on anesthetized rabbits to determine the effect of internal carotid artery injection of various contrast media on permeability changes in the blood-brain barrier. Changes in respiratory pattern, neuromuscular effects, and trypan blue extravasation were recorded after 3-ml injections of ionic and nonionic contrast media. Metrizamide and iothalamate meglumine were compared at iodine doses of 300, 400, and 500 mg I/ml. Metrizoate at 280 and 440 mg I/ml and diatrizoate meglumine at 385 mg I/ml were also included for comparison. The results demonstrated that metrizamide at all three iodine concentrations used caused minimal disruption of the blood-brain barrier, the effect being no greater, statistically, than saline controls. Iothalamate was benign at the lowest iodine concentration, but caused significant barrier breakdown at the two higher concentrations. These results suggest that alterations in blood-brain barrier permeability following angiography are mediated by both hyperosmolality of the contrast medium and the chemotoxicity of the contrast molecule.