SUMMARY We measured the concentration of radon from sea level to 5,000 meters under different meteorological conditions. Assuming that the radon is in equilibrium with its daughters we can calculate the radon concentration from that of the daughter products. pnd The sampling is carried out by aircraft with two different techniques. 1. Stepwise sampling - The activity of the filter at the end of each sampling gives the radon concentration at the designated altitude. 2. Continuous sampling during climb and descent - A curve giving the activity of the filter as a function of altitude is analyzed with a computer. In most cases the variations are most marked in the lower levels, the activity then falls off slowly with altitude. Results can be interpreted ir terms of the meteorological conditions corresponding to the different samples.