A global three-dimensional transport model of the atmosphere, having a grid resolution of 7.83¿ (latitude)¿10¿ (longitude)¿19 layers in the vertical and driven by European Centre for Medium Range Weather Forecasts (ECMWF) wind field analyses of 1990, is used to simulate the distribution of natural (210Pb, 7Be, 10Be) and artificial (90Sr), radioactive, water-soluble, aerosol-borne tracers. Because of their well-known source-sink distribution these tracers build an ideal tool to depict transport processes in the whole atmosphere and to test the models ability of reproducing these. In particular, this paper focuses on mass exchange between stratosphere and troposphere, using the concentration ratio 10Be/7Be as an indicator. In general, the agreement between observations and model results is quite good, except in polar regions. Modeled arctic 210Pb and 7Be concentrations are overestimated and the annual cycle is underestimated. The modeled antarctic annual cycle of 10Be/7Be shows maxima in winter and spring, whereas the observations exhibit a summer maximum. These discrepancies are attributed to deficiencies in ECMWF wind fields, to differences between observed and model used precipitation, and to the employed parameterization schemes of dry and wet deposition which may be inadequate in the cold polar regions. ¿ American Geophysical Union 1995