the purpose of this study was to determine whether the ingestion of an energy-free aspartame beverage (asp) (99 mg aspartame/l) would influence endurance performance relative to a water (wat) trial. nine trained male cyclists underwent two cycle ergometer trials to exhaustion at 75% vo2 max. experimental beverages were ingested every 15 minutes beginning 45 minutes prior to cycling. mean (± sd) exercise times to exhaustion were 100 ± 28 minutes for the asp trial and 99 ± 34 minutes for the wat trial (p>.05). respiratory exchange ratio and heart rate did not differ in response to the treatment condition. ratings of perceived exertion were significantly elevated in the asp trial but only at the initiation of exercise. it was concluded that the asp beverage did not impair endurance performance relative to wat.