Duchenne muscular dystrophy (DMD) is a genetic disease caused by defects in the dystrophin gene resulting in loss of the structural protein dystrophin. Patients have reduced diaphragm functional capacity due to progressive muscle weakness. Respiratory morbidity in DMD is further characterised by hypoxaemic periods due to hypoventilation. DMD patients die prematurely due to respiratory and cardiac failure. In this study, we examined respiratory function in young adult male mdx (dystrophin deficient) mice (C57BL/10ScSn-Dmd(mdx)/J; n¿=¿10) and in wild-type controls (WT; C57BL/10ScSnJ; n¿=¿11). Breathing was assessed in unrestrained, unanaesthetised animals by whole-body plethysmography. Ventilatory parameters were recorded during air breathing and during exposure to acute hypoxia (F(i)O(2)¿=¿0.1, 20 min). Data for the two groups of animals were compared using Student's t tests. During normoxic breathing, mdx mice had reduced breathing frequency (p¿=¿0.011), tidal volume (p¿=¿0.093) and minute ventilation (p¿=¿0.033) compared to WT. Hypoxia increased minute ventilation in WT and mdx animals. Mdx mice had a significantly increased ventilatory response to hypoxia which manifest as an elevated % change from baseline for minute ventilation (p¿=¿0.0015) compared to WT. We conclude that mdx mice have impaired normoxic ventilation suggestive of hypoventilation. Furthermore, mdx mice have an enhanced hypoxic ventilatory response compared to WT animals which we speculate may be secondary to chronic hypoxaemia. Our results indicate that a significant respiratory phenotype is evident as early as 8 weeks in the mdx mouse model of DMD.