The relative contribution of aircraft emissions to the total atmospheric NO x content is studied by means of the atmosphere general circulation model ECHAM, which was extended by a simplified (linear) NO x chemistry module with NO x and HNO 3 as prognostic variables. NO x originates from fossil fuel combustion, biomass burning, soil microbial activity, lightning, degradation of N 2 O in the stratosphere, and aircraft emissions. NO x and HNO 3 are removed by dry deposition, HNO 3 additionally by wet deposition.A simulation in seasonal cycle mode was performed. Both the resulting January and July mean distributions of NO x and the relative contributions of the different sources to the total atmospheric NO x burden are analysed, including their uncertainty ranges. NO x emissions from aircraft cause a significant change to the background NO x concentration. During January more than 60% of the NO x emissions found in the region between 30 and 60°N, and 175-325 hPa arise from aircraft. The maximum value in the North Atlantic flight corridor is found to be larger than 85%. During July, aircraft emissions contribute only about 20% to the NO x burden in this region. In January and July the uncertainty ranges of these contributions are 42-80% and 10-37%, respectively.