Closed fenestration wounds in four mongrel dogs were used to study the source of fibroblast proliferation and extracellular matrix production during healing; the arrangement and attachment of newly formed collagenous fibers; and the cementogenesis and osteogenesis at healing sites. Fenestration wounds were made through the alveolar bone, periodontal ligament, cementum, and dentin, and citric acid, tetracycline, or sterile water was applied to the dentinal walls for 3 minutes. Nonresorbable membranes were randomly placed over half of the defects. Animals were killed at 1, 3, 7, or 21 days and routine histologic examinations with hematoxylin and eosin staining followed. Results of this pilot study suggest that the periodontal ligament and/or alveolar bone are the main source of fibroblast proliferation and migration as well as extracellular matrix formation at the initial stages of healing, and that at 21 days, citric acid stimulated more cementogenesis than tetracycline or sterile water. Also, while the tetracycline influenced the maximal deposition of alveolar bone, no differences in healing were found between the citric acid, tetracycline, and sterile water with and without the use of membrane barriers.