Porcine visceral and extremity arteries were embolized with detachable silastic balloons; 10 balloons were filled with isoosmolar metrizamide, five were filled with 30% megluminesodium diatrizoate and five were filled with 3:1 mixtures of blood and 76% meglumine-sodium diatrizoate. Follow-up radiographs were obtained weekly for 2 weeks, at 4 weeks and at 5 months. At 3 months, arteriography was performed. All balloons occluded the vessel completely upon detachment. Balloons filled with metrizamide did not change their volume or radiopacity during the entire observation period and all remained inflated. The balloons filled with diatrizoate salts enlarged by osmotic growth during the first two weeks and there was a decrease in opacity. Thereafter, one balloon filled with 30% diatrizoate salt solution and one filled with blood-76% diatrizoate deflated. In the former, the embolized artery remained occluded, while in the latter, the vessel reopened and the balloon was found incorporated into the vessel wall at the site of detachment. It is concluded that isoosmolar metrizamide is an ideal filler for detachable silastic balloons since it does not lead to changes in volume or radiopacity and also guarantees long-term vascular occlusion.