The aim of this study was to evaluate the efficacy of two bioresorbable barriers especially produced for guided bone regeneration. Six beagle dogs were used in this investigation. At the beginning of the study, all mandibular premolars were extracted and after a healing period of three months three screw-type machined implants were inserted in each side of the mandible. At the buccal aspect of each implant bed a dehiscence type defect with a coronoapical and mesiodistal extension of 5 mm was created. Then, one of the following four methods for defect treatment was applied: 1) guided bone regeneration (GBR) with a bioresorbable barrier (poly-L/DL-lactide) (test group 1), 2) GBR with a bioresorbable composite barrier [poly-L/DL-lactide reinforced with tricalciumphosphate (TCP)] (test group 2), 3) GBR with an expanded polytetrafluoroethylene barrier (GTAM) (control group 1), and 4) no treatment (control group 2). The animals were sacrificed six months after implant installation. During the healing period the most common problems encountered in the test groups 1 and 2 were soft tissue dehiscences and bioresorbable membrane exposures. Retrieved tissue specimens were processed for histological and histometric evaluation. The mean amount of vertical bone regeneration at the defect areas was 1.75 mm for test group 1, 1.82 mm for the test group 2, 2.38 mm for control group 1, and 1.93 mm for control group 2. When the amount of bone regeneration in contact to the implant surface was evaluated, the regeneration result was 1.45 mm for test group 1, 1.49 mm for test group 2, 2.08 mm for control group 1, and 0.91 mm for control group 2. For both measurements, no statistically significant differences could be observed between the different groups. The present animal experiment showed that the treatment of dehiscence type defects around machined implants with the two bioresorbable barriers showed only slightly superior results compared to the treatment without barriers (control group 2). The best results--although not statistically significant different from the other treatment groups--were obtained when nonresorbable barriers were used.