Oxygen uptakes and running speeds of two marathon runners as well as average slopes of the running terrain were measured approximately every 3 miles of a competitive marathon. The runners had energy expenditures requiring from 68 to 100% of their Vo2 max, confirming previous estimates. As the race progressed, both runners exhibited decreases in V92 while running at similar rates and slopes. At 23.4 miles, one runner was found to be utilizing 4.54 l/min, a value equivalent to his Vo2 max.