The physical fitness of healthy young men of an arctic population of Indians was studied, using two types of physiological measurements during muscular work. In one series of experiments the respiratory gas exchange and heart rate were determined during apparently ‘steady-state’ exercise on an ergometer bicycle and the maximal O2 intake was estimated. In another series the response of pulmonary ventilation to a standard exercise load was measured on the same bicycle and the time to recuperate was determined, as well as the extraventilation caused by the exercise. Comparisons were made with results observed on a group of sedentary-living young men and champion athletes drawn from the population of Norway. The Indians' fitness to work occupies a somewhat intermediate position between the sedentary subjects and the athletes. Submitted on August 3, 1959