['To determine the effect of the Breathe Right (BR) external nasal dilator strip on treadmill exercise performed while wearing an upper maxillary mouthguard.', 'Two-way repeated-measures design with subjects acting as their own controls. Subjects performed 2 randomly assigned bouts of incremental treadmill exercise (with and without the BR strip) while wearing upper maxillary mouthguards.', 'Nineteen young, healthy, recreationally active men.', 'we assessed subjective nasal patency levels at rest. We also recorded heart rate, dyspnea rating, and treadmill speed at 2 submaximal exercise levels and at volitional fatigue.', 'Subjective nasal patency was significantly increased with the strip. Repeated-measures analyses of variance revealed a significant main effect of the BR strip on dyspnea ratings during exercise, but there was no effect of the strip on test duration, heart rate, or running speed during the tests.'] Do external nasal dilator strips affect treadmill performance in subjects wearing mouthguards?