Liquid polymers have previously been used to treat experimental and human aneurysms. However, the delivery of a liquid embolic material into the cerebral circulation involves a high risk of irreversible vessel occlusion and stroke. To evaluate methods for the safe and effective treatment of experimental aneurysms with liquid polymer injection, we tested four different techniques to deliver cellulose acetate polymer (CAP) or N-hexyl-cyanoacrylate into canine side-wall carotid artery aneurysms. The animals were observed for 1 to 10 weeks after treatment. Two aneurysms were treated without protection of the distal circulation, one with CAP and another with N-hexyl-cyanoacrylate. In four cases, an angioplasty balloon was inflated within the parent artery during endosaccular injection of CAP. In two of these cases, the balloon was placed adjacent to the aneurysm orifice, resulting in simultaneous occlusion of both the aneurysm and the parent artery, and in the other two cases, the balloon was positioned proximal to the aneurysm, resulting in temporary flow arrest. Three aneurysms were treated with either CAP or N-hexyl-cyanoacrylate after implantation of a balloon-expandable tantalum stent within the parent artery across the aneurysm orifice. Complete angiographic obliteration was achieved in all but one case. One aneurysm ruptured. Another partially occluded aneurysm reopened 10 weeks after treatment. In all cases treated without stents, distal migration of the polymer resulted in either stenosis or occlusion of the parent arteries. The combination of stent implantation and polymer injection resulted in permanent aneurysm occlusion without detectable polymer migration. An intravascular stent deployed within the parent artery across the aneurysm orifice acted as a safety net during endosaccular polymer injection by allowing blood to flow from the aneurysm cavity while preventing distal migration of liquid polymer.