The purpose of this study was to examine serum metabolites, cardiorespiratory parameters and creatine kinase during running on 3 consecutive days. Thirteen trained marathon runners exercised to exhaustion on a treadmill at 85 +/- 3% VO2max on each day. Expired gases and blood samples were obtained at rest, after 10 and 30 min of exercise and at exhaustion. There were no significant differences over days for glucose, insulin, lactate, free fatty acids, creatine kinase, oxygen uptake, minute ventilation, heart rate, rating of perceived exertion, respiratory exchange ratio or run times. Serum glycerol was elevated (p less than 0.05) both at rest and during exercise on each successive day. The findings suggested that, except for serum glycerol, acute metabolic and cardiorespiratory responses to exhaustive aerobic exercise are not altered by 3 days daily repetition by endurance trained individuals.