A specially designed silicone balloon, expanded in vivo with normal saline, was tested for possible use as a retinal tamponade in desperate cases of rhegmatogenous retinal detachment. In six pigmented rabbits there were extensive postoperative complications, but in eight owl monkeys results were noticeably better. When the seam of the balloon was in contact with the retina there were adverse effects; seamless areas of the balloon did not appear to disturb the retina. Histopathologic studies indicated that excessive expansion of the balloon can cause significant damage, but that carefully controlled expansion can probably minimize complications. The presence of the balloon appears to alter electroretinographic response, but without necessarily causing histopathologic changes. Balloons of this type have been used in treatment of a small number of otherwise inoperable cases of retinal detachment with promising results.