Objectives: The aim of this study was to histomorphometrically evaluate the effects of the association of platelet rich fibrin (PRF) with a xenograft, in critical bone defects in rabbits' calvaria. Materials and methods: Twelve New Zealand adult male rabbits were distributed in two groups, according to the graft material that was used: Control (CG) - Bio-Oss xenograft (n=6); and Test (TG) - Bio-Oss xenograft associated to PRF (n=6). In all animals two bilateral bone defects with twelve millimeters in diameter were created, but just one defect received the Bio-Gide collagen membrane coverage. After eight weeks the animals were sacrificed and a histomorphometric analysis was done. PRF Obtention: Following Choukroun's original protocol (Dohan et al., 2006), blood samples were centrifuged for ten minutes in a 3,000 rpm spin, by the Intra-Spin L-PRF centrifuge (Intra-lock, USA). The PRF was cut in small pieces before being associated with the xenograft for TG. Surgical Procedure: A full thickness flap was performed, two bone defects were created (with a trephin bur) and the grafts placed inside the defect. After the membrane coverage (just in one side), the flaps were sutured with nylon thread. Histomorphometric Analysis: After histological processing, images were captured using an optical microscope coupled to a digital camera, and reproduced on a computer screen using the Infinity Analyze software (Lumenera, Canada). Histomorphometric analysis was performed by evaluating the following tissues: Non Vital Mineralized Tissue (NVMT), Vital Mineralized Tissue (VMT) and Non Mineralized Tissue (NMT). Statistical Analysis: The intra and intergroup statistical comparison was done by Kuskal-Wallis test, using a p value of 0.05. Results: The histomorphometric results showed, for the side with membrane coverage, no statistical significant difference between CG and TG for all parameters. On the other hand, for the side without membrane coverage, a statistical significant difference between CG and TG was observed for VMT and NMT. Concerning the side with membrane coverage, TG and CG had 17.88 ± 7.57% and 13.50 ± 0.09% for NVMT, respectively (p=0.2971). For VMT, TG and CG had 13.88 ± 5.68% and 13.29 ± 0.16%, respectively (p=0.5211). For NMT, TG and CG had 68.24 ± 5.05% and 73.21 ± 0.12%, respectively (p=0.0776). Regarding the side without membrane coverage, TG and CG had 16.59 ± 3.54% and 13.09 ± 0.06% for NVMT, respectively (p=0.0542). For VMT, TG and CG had 9.52 ± 0.60% and 6.56 ± 0.08%, respectively (p=0.0039). For NMT, TG and CG had 73.74 ± 3.48% and 80.34 ± 0.05%, respectively (p=0.0038). Conclusions: In this experimental model: 1) Platelet rich fibrin does not result in higher levels of bone formation when a guided bone regeneration technique (i.e. with membrane coverage) is used. 2) The usage of the collagen membrane has a synergistic effect on bone healing when associated with a xenograft. 3) Platelet rich fibrin may increase the level of newly formed bone only in bone grafting procedures using xenograft without collagen membrane coverage. Schlagwörter: platelet rich fibrin, bone regeneration