The blood glucose response of a male ultramarathon runner was monitored throughout a 1005-km race. Before the race the runner had a fasting blood glucose concentration of 5.1 mM. At no stage during the race were his mean blood glucose levels less than 5.8 mM. This was partially attributed to the eating patterns of the athlete, the times at which blood samples were taken, the glycaemic index of food ingested and hyperglycaemia. While there was no evidence of glucosuria, ketones were present in the urine on one day of the event. There were other signs suggesting that at various stages of the event the runner had a metabolic acidosis. Possible reasons for this are discussed.