The aim of these experiments was to investigate the effect of providing substrate in the form of CHO or fluid as water or a dilute glucose-electrolyte solution on the metabolic and circulatory changes during prolonged endurance exercise. It was hoped to establish the relative importance of fluid or substrate replacement in promoting endurance capacity. Subjects exercised significantly longer on a cycle ergometer when the glucose-electrolyte solution was given compared with the administration of large amounts of glucose or fructose in the form of polymer solutions. The effectiveness of the glucose-electrolyte solution in prolonging exercise time to exhaustion was not shown during exercise at high (33oC) and low (2oC) ambient temperatures. Fluid balance does not seem to be a priority during exercise at low ambient temperatures and exercise time in the heat was too short for the possible benefits of fluid replacement to occur. During 2h of moderate exercise (50% VO2max) at a high ambient temperature, the ingestion of the glucose-electrolyte solution was associated with the maintenance of plasma volume and minimal physiological disturbances which may limit performance. There was a tendency toward greater rectal temperatures and higher heart rates during rehydration with a hypertonic glucose polymer solution. The effect of fluid and substrate replacement during exercise performed over a range of intensities (50 - 70% VO2max) was investigated. It was suggested that the ingestion of a glucose polymer solution, in an attempt to provide glucose to the working muscle, may compromise fluid balance. The usefulness of carbohydrate feeding during prolonged exercise of moderate intensity where thermoregulation is a priority is questionable. The maintenance of plasma volume through the replacement of fluid losses during exercise seems to be the priority in the promotion of endurance capacity rather than the provision of carbohydrate.