Blood was drawn from cross-country skiers at 1–3 min after the finish in competitions on distances from 10 to 85 km and the blood lactate determined. Despite a maximal effort of the skiers, accentuated at the end of the race, there was a successive decrease in the blood lactate concentration with work time. After a 10-km race, work time 35–36 min, the average was 139 mg/100 ml of blood (12.5 mEq/liter); after a 30-km race, with a time of 1 hr 50 min-1 hr 56 min, the mean value was 68 mg/100 ml (6.1 mEq/liter); and after a 50-km race, work time 3 hr 6 min-3 hr 18 min, 39 mg/100 ml (3.5 mEq/liter). A lactate concentration exceeding 100 mg/100 ml is a common finding after maximal muscular exercise involving large muscles. The explanation for the low values after prolonged maximal work, indicating a different kind of fatigue, is presently obscure. Data are presented on the oxygen uptake attained during skiing at actual racing speed (average Vo2 = 4.45 liters/min). Submitted on July 16, 1962