Objective: There is growing speculation that the impairment in regulatory-T-cell (Treg)-mediated dominant tolerance may play an important role in the pathogenesis of allergic rhinitis (AR). The aim of this study was to investigate whether the changes in the forkhead transcription factor 3 (Foxp3) gene expression may aggravate nasal mucosal inflammation in allergic mice, and whether or not these features result from the loss of Tregs. Methods: AR was induced by both intraperitoneal injection and intranasal administration of ovalbumin in BALB/c mice, while the control mice were treated with saline. A comparison of the frequency of CD4(+)CD25(+)Foxp3(+) Tregs in peripheral blood mononuclear cells of the AR and control mice was made by flow cytometry. Spleen mononuclear cells were used for RNA extraction and RT-PCR was used to measure Foxp3 mRNA expression. Results: The expression of the Foxp3 gene was significantly reduced in spleen mononuclear cells in AR mice compared with the control. Moreover, a significant decrease in the percentage of CD4(+)CD25(+)Foxp3(+) Tregs was exhibited in peripheral blood mononuclear cells of AR mice compared with the control mice. Conclusion: The insufficiency of Tregs and the Foxp3 gene may contribute to the development of AR in mice. Copyright (C) 2009 S. Karger AG, Basel