The effects of a competitive marathon race on serum biochemical and haematological parameters have been evaluated. Blood samples were obtained shortly before and immediately after the race; urine samples were also obtained before and after the race. Body weight was recorded pre- and post-race. During the race subjects consumed a total of 1.41 of either water or a dilute glucose-electrolyte solution. The average weight loss of the runners was 2.09±0.77 kg (mean ± SD), corresponding to 2.9±0.8% of body weight. Small but significant increases in both haematocrit and haemoglobin concentration occurred; plasma volume was calculated to decrease by 4.7%. Serum potassium concentration showed no change, but the response was highly variable; serum sodium concentration increased in line with the decrease in plasma volume. In the group of subjects drinking water during the race, the pre-race plasma glucose concentration was 5.3±1.2 mmol·l−1, this was unchanged after the race (5.0±1.2 mmol·l−1). A significant increase (P<0.01) in the plasma glucose concentration, from 5.2±0.6 to 6.0±1.5 mmol·l−1 occurred in the group of subjects drinking the glucose-electrolyte solution. Apart from this, there were no significant differences between the two groups.