Some of the effects of acute fat loading on arterial O2 at high altitude are described. Five dogs were taken to 12,000 feet in an altitude chamber and given fat infusions (1 gm/kg). There was a 10% drop in the arterial O2 content and a similar drop in the O2 capacity. One of the dogs had a surgically induced intra-atrial septal defect and died of fat emboli. Three dogs serving as controls at ground level had a 4% drop in arterial O2 content after fat infusions. A practical method for screening poor human risks among flying personnel, employing a fat tolerance test, is discussed. The drop in the O2 content and capacity of canine blood after fat infusions at 12,000 feet is at least twice that at ground level. Submitted on July 13, 1959