Although mouthguards have been found to reduce injury, many athletes choose not to use them because of the belief of negative effects on breathing with mouthguard use. Therefore, this study investigated the effects of mouthguard use on physiological variables in women using a self-adapted mouthguard made for women and a universal self-adapted mouthguard. Eleven subjects (mean [+ or -] SD; age: 22 [+ or -] 3.3 y; height: 159.8 [+ or -] 4.3 cm; body mass: 63.7 [+ or -] 8.9 kg) performed 3 separate maximal cycle ergometer tests with either 1 mouthguard or no mouthguard. Heart rate (HR), rating of perceived exertion (RPE), oxygen consumption ([[??].o.sub.2]), minute ventilation ([[??].sub.E]), and respiratory exchange ratio (RER) were measured at each exercise intensity and at the end of each test. HR, RPE, and [[??]o.sub.2] increased for each mouthguard condition across power levels, [[??].sub.E] and RER increased more from 110W to max with the no mouthguard condition than for either mouthguard. However, there was no significant difference in bE or RER at any given power level between mouthguard conditions. The results indicated mouthguard use did not affect HR, RPE, [[??].sub.E], [[??]o.sub.2], or RER at any given power level during exercise, including the guard made specifically for women. Therefore, athletes are encouraged to use mouthguards without fear of negative aerobic performance effects. KEY WORDS oxygen consumption, ventilation, cycle ergometer, RER, RPE