This study evaluated the effects of guided tissue regeneration (GTR) in the treatment of Class II furcation defects in beagle dogs. Gore-Tex periodontal material was used, and the amount of furcation fill and the surface area corresponding to new connective tissue attachment and new bone were evaluated. If present, epithelium was also evaluated. Six female dogs with naturally-occurring periodontitis were given thorough root debridement. Four weeks later mucoperiosteal flaps were raised, involving the mandibular 2nd, 3rd, and 4th premolar and 1st molar teeth. After debridement, notches were placed on the roots at the level of furcal alveolar bone. Gore-Tex material was adapted to the furcation and secured with sutures on two teeth of each quadrant. The flaps were sutured tightly, making sure the margin of the material was covered. Teeth receiving only surgery acted as controls. Gore-Tex material was removed 1 month following surgery. All dogs were sacrificed by exsanguination at 3 months. Mesiodistal sections were evaluated by descriptive histology. Surface area determination of the furcal tissues was carried out using a microscope attached to a computerized digitizer. Approximately 10 serial sections per tooth were evaluated. Mean values for each dog were obtained for experimental and control teeth and analyzed using paired t-tests (N = 6) to see whether different treatments had any effect on the parameters tested. Different degrees of fill were seen with epithelium, new connective tissue, and bone. Statistically, GTR gave significantly better results in the amount of connective tissue and bone fill achieved.