We have measured with aircraft the radon concentration to 5,000 meters under different meteorological conditions in the Southwest of France above Landes and Armagnac=Bigorre not far from the Atlantic Ocean. We assume that the radon is in equilibrium with its descendants and calculate its concentration from that of the daughter products. The apparatus has been designed so that it can be used with aircraft or with captive balloons. IIe PRINCIPLES OF THE METHOD OF RADON MEASUREMENT Airborne aerosols are collected on a filter by a pump and the activity is measured during sampling. wre With the aircraft we use two techniques. The first consists in sampling at a fixed altitude (sampling time is a function of the activity and of the desired precision). The filter which collects the radioactive faughter products is counted during the sampling and is changed at each altitude. The concentration is thus determined directly for each level and it is possible to cover a considerable range of altitudes in a few hours. In the second procedure we take the samples continuously during climb or descent of the aircraft without changing the filter, the activity being measured continuously. This method allows studying the activity at different altitudes in relatively short time but it is necessary to analyze the activity curve to obtain the varlations of radon concentration. In both cases we make simultaneous measurements at the surface. With the captive balloons we place several samplers along the cable with one at the surface. The activities of the filters and the meteorological data are transmitted by telemetry.