Athletes competing in contact sports commonly wear intra-oral dental mouthguards. Data are sparse concerning the influence of a mouthguard on breathing during exercise. We compared VE and VO2 during submaximal and maximal exercise on a skating treadmill (TM) while wearing an intra-oral dental mouthguard. Female varsity hockey players (n = 12) performed two skating tests on a TM with and without a mouthguard (WIPSS Jaw-Joint Protector). The players wore the mouthguard during hockey practices prior to collection of ventilation data on the treadmill. Also, the players completed a questionnaire that examined their perception of the mouthguard in terms of ventilation, comfort and performance. A 10-point rating scale was used for this evaluation. Two performance tests on the skating treadmill examined the effect of the mouthguard on submaximal and maximal aerobic exercise. The subjects skated for 4 min at 2 submaximal velocities (14 and 16 km h-1), separated by 5 min of passive recovery. A VO2max test followed the submaximal tests and commenced at 18 km h-1 with the velocity increasing by 1 km h-1 every minute until volitional fatigue. VE, VO2, VCO2 and RER were analyzed using a Sensor Medics 2900 metabolic cart. Ventilation was unchanged when skating at the two submaximal velocities. VC2max was 48.8 ml/kg.min using the intra-oral mouthguard and was 52.4 ml/kg.min without a mouthguard. VEmax was 108.5 L/min using the intra-oral mouthguard and was 114.1L/min without a mouthguard. The results showed that VEmax and VO2max were lower using the mouthguard compared to the no-mouthguard condition.