Damage inflicted on the oral mucosa may result in the development of an unfavorable appearance, functional disturbances, or growth retardation in the buccal-oral cavity region. In an attempt to minimize such disturbances, transplantation of the free oral mucosa was investigated. Changes in the vascular picture were studied by the intravascular injection of chloropercha. Three-dimensional and morphological observations of blood vessels were supported by macroscopical and pathohistological studies. The animals used were 60 adult dogs weighing about 4-8 kg raised in similar conditions. The left and right buccal mucosal flaps and the left and right maxillary third incisors served as the material. Intravenous anesthesia was used. In Exp. 1: a 1.0 x 1.0 cm piece was taken from the left buccal mucosa. A mucosal graft of the same size was obtained from the right buccal mucosa (used for the free buccal mucosa transplantation). In Exp. 2: the right buccal mucosa from which the mucosal flap had been taken was left as an open wound. In Exp. 3: a gingival section (1.0 x 1.0 cm) was taken from both sides of the maxillary gingiva at the level of the third maxillary incisors. A free buccal gingival graft was prepared from the left side and transplanted to the open wound on the right. The free buccal gingival graft from the right side was transplanted to the open wound on the left. 1) In the free buccal mucosal transplantation to the buccal mucosa, slight intrusion of blood vessels into the transplant was obvious 2 days after transplantation. By the fifth day many blood vessels had developed, by the sixth day these blood vessels formed a loop in the epithelial region, by the fourteenth day the arrangement of blood vessels had become well arrayed, and by the twentieth day blood vessels gave the same appearance as vessels in the transplantation bed. 2) Transplantation of the free mucosa on to the buccal mucosa of the opposite side gives better healing than leaving the damaged buccal mucosa as an open wound. 3) With transplantation of free gingiva to the gingiva of tooth neck, blood vessels had partially advanced into the transplant by the fifth post-operative day. By the tenth day, loop formation of vessels on the gingival epithelium together with gingival adhesion were well arranged. By the fortyfifth day, blood vessels in the transplant presented a similar picture to the normal. 4) Gingival transplantation to gingiva of the tooth neck gave better healing than buccal mucosal transplantation. Moreover, the appearance after operation was more satisfying. 5) The pathohistological findings for this vascular picture agreed in general with the macroscopic appearance.