Trypan blue and 197HgCl2 were used as markers of the increased leakage through the blood-brain barrier which developed after selective injection of different contrast media formulations into the internal carotid arteries of rabbits. Interest was focused on a new, experimental, nonionic medium (C-29) and possible effects resulting from the addition of Ca2+ and Mg2+ to nonionic contrast media. Using 197Hg, it was possible to quantitate the increase in extravasation produced by the contrast media. C-29 caused less damage than either metrizamide or meglumine iothalamate. The addition of Ca2+ and Mg2+ to nonionic contrast media seemed to stabilize the blood-brain barrier.