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To induce artificial mouth breathing on experimental animals has extremely been difficult because of its immediate death following nasal sealing treatment, and we could find only an imitating experiment reported by Butcher et al (J. Periodont. 32: 38, 1961). The purpose of this study is to induce absolute mouth breathing in rats and to investigate histopathological changes of their gingival tissues. Thirty six of Wister strain rats were equally divided into control and experimental groups. To induce absolute mouth breathing artificially, the nose of all experimental animals were sutured using thin wire and metallic buttones. After two weeks of experimental period, animals were sacrificed and materials were obtained.Consequently, we could find much more remarkable hyperkeratinization of epithelial layers in the experimental animals than the control. Inf lammatoric changes, however, were found in both groups of animals, but less in control. As the conclusion of this investigation, absolute mouth breathing may induce hyperkeratinization of the epithelial tissues, but does not initiate gingival inflammation following two weeks of experiment.