To investigate the effect of sucrose or caffeine ingestion on the performance of prolonged running, five male distance runners attending senior high school (15.6 yrs) carried out running on a treadmill at an intensity corresponding to the individuals' 80% VO2 max until exhaustion. Before and 45 min after exercise, the subjects were given either a placebo (Con), sucrose (81 +/- 18 g) (Su), caffeine (384 +/- 13 mg) (Caf), or sucrose (72 +/- 22 g) plus caffeine (396 +/- 29 mg) (Su + Caf) solution. The duration of the exercise was significantly longer in Su, Caf, and Su + Caf than in Con. The duration in four of five subjects was longest in Su + Caf, although it was not significantly different from that in Su or Caf. Carbohydrate (CHO) utilization was highest in Su while fat utilization was highest in Caf. The energy supply from both sources was almost the same between Con and Su + Caf. The plasma glucose concentration was higher in Su than in Con. The plasma free fatty acid (FFA) level was higher in Caf than in Con. The plasma glucose and lactic acid concentrations were highest in Su + Caf while the plasma FFA level was the same as in Con. In conclusion, ingestion of sucrose, caffeine, or sucrose plus caffeine solution was equally effective in improving endurance during running carried out at an intensity of approximately 80% VO2 max.