Flow rates of two general aviation constant flow oxygen systems were tested at altitude using a Ted Nelson flow meter and using timed flow into a volume bag. Contrary to the manufacturer's claim, the Rajay SK-10 system was not altitude compensated and provided a flow rate of about 2.2 1/min independent of altitude. The Scott system provided the stated 2.5 l/min with the standard connector and 8 l/min with the "green" connector, the significance of the color code being not widely understood. The conclusions of the 1975 report on oxygen mask performance should be revised according to these flow data. The Ted Nelson flow meter performed within its stated accuracy.