Oral respiration associated with an obstructed nasal airway is common in orthodontic patients. For several years chronic oral respiration has been implicated as a prime causative factor in the development of "adenoid facies or the "long-face syndrome. The animal experiment reported here begins a series designed to study, as separate variables, the 2 components of chronic oral respiration: (1) chronic absence of active nasal respiration and 2) chronic mouth opening to find out what dentofacial changes can be attributed to chronic absence of active nasal respiration alone. In this pilot study, 5 growing dogs underwent tracheotomy so that significant active nasal respiration was not possible and oral respiration was not essential.