PURPOSE: Dimethylglycine (DMG) is a dietary supplement that is advertised to aid in the delivery of oxygen during aerobic exercise. We examined the effects, of DMG on the maximal exercise performance of well-trained college women. We hypothesized that pre-exercise DMG consumption would lead to an increase in V̇o2max. METHODS: Nine well-trained college women (age = 20 ± 1 y, height = 165.2 ± 4.5 cm, body mass = 63.6 ± 5.6 kg, V̇o2max = 49.4 ± 2.8 ml/kg/min were recruited as subjects. Subjects ingested three, 800mg doses of DMG or a placebo (PLC) on two occasions in a randomized, double-blind fashion. Thirty minutes after the last dose subjects performed graded, maximal treadmill exercise to exhaustion. V̇o2, heart rate, perceived exertion, and time to exhaustion were compared between conditions. RESULTS: No significant differences were found using independent t-tests in the DMG versus the placebo trials on maximum heart rate (DMG 191.4 ± 7.5 bpm PLC 191.2 ± 6.1 bpm, p = 0.79), V̇o2max (DMG 48.8 ± 2.6 ml/kg/min PLC 49.2 ± 2.6 ml/kg/min, p = 0.56), and exercise time (DMG 12.81 ± 42 s PLC 12.84 ± 0.45 min, p = 0.81). Further, during submaximal exercise stages, heart rate (p = 0.52, perceived exertion (p = 0.48), and respiratory exchange ratio (p = 0.65) were not different between treatments. CONCLUSION: Based on these results, it can be concluded that DMG has no significant effect on improving the endurance capacity of well-trained women. Practical Applications: There is no evidence to recommend DMG as an ergogenic aid to endurance athletes.