The aim of this investigation was to test the hypothesis that membrane permeability is necessary in bone formation using the principle of guided tissue regeneration. On the forehead of 8 rabbits, titanium test cylinders were anchored in the calvaria. These cylinders were either covered by an expanded polytetrafluoroethylene (ePTFE) membrane generating a chamber for bone formation or they were sealed off by cast titanium. The implanted cylinders were covered by resuturing the periosteum and the cutaneous flap. After 8 months of healing, new bone had formed in all cylinders in all animals irrespective of whether the chamber for bone formation was sealed off by cast titanium or the ePTFE membrane. Based on these results, we conclude that permeability of the membrane is not necessary in the guided generation of new bone.