This study determined the effects of fluid and carbohydrate ingestion on performance, core temperature, and cardiovascular responses during intense exercise lasting 1 h. On four occasions, eight men cycled at 80 +/- 1% (+/- SEM) of VO2max for 50 min followed by a performance test. During exercise, they consumed either a large volume (1330 +/- 60 ml) of a 6% carbohydrate (79 +/- 4 g) solution or water or a small volume (200 +/- 10 ml) of a 40% maltodextrin (79 +/- 4 g) solution or water. These trials were pooled so the effects of fluid replacement (Large FR vs Small FR) and carbohydrate ingestion (CHO vs NO CHO) could be determined. Performance times were 6.5% faster during Large FR than Small FR and 6.3% faster during CHO than NO CHO (P < 0.05). At 50 min, heart rate was 4 +/- 1 b.min-1 lower and esophageal temperature was 0.33 +/- 0.04 degrees C lower during Large FR than Small FR (P < 0.05) but no differences occurred between CHO and NO CHO. In summary, Large FR slightly attenuates the increase in heart rate and core temperature which occurs during Small FR. Both fluid and carbohydrate ingestion equally improve cycling performance and their effects are additive.