Twenty-one embolizations were performed in 12 swine using a detachable silicone balloon-catheter system. In 11 embolizations, balloons were filled with 60% meglumine sodium diatrizoate (R-60); 30% meglumine sodium diatrizoate (R-30) was used in 10 embolizations. Serial radiographs were obtained to determine the duration of balloon inflation. Repeat angiography and autopsy examinations were performed three to six months following embolization. Eighty percent of balloons filled with R-30 were still inflated after 10 days, as opposed to 27% of the balloons filled with R-60. Balloons which remained inflated for more than 10 days caused permanent occlusion of the embolized arteries. The vessels were occluded precisely at the point at which the balloons were initially attached. Pathologically, the occluded vessels exhibited organized thrombi with minimal inflammation. Detachable balloons appear to be an effective, safe method of performing selective vascular occlusion in many circulatory systems. Careful control of the volume and osmolarity of the contrast medium used to inflate the balloons produces permanent arterial occlusion.