The Hamburg atmospheric general circulation model (AGCM) ECHAM-4 is used to identify the main source regions of precipitation falling on Greenland and Antarctica. Both water isotopes H218O and HDO are explicitly built into the water cycle of the AGCM, and in addition the capability to trace water from different source regions was added to the model. Present and LGM climate simulations show that water from the most important source regions has an isotopic signature similar to the mean isotope values of the total precipitation amount. But water from other source regions (with very different isotopic signatures) contributes an additional, non-negligible part of the total precipitation amount on both Greenland and Antarctica. Analyses of the temperature-isotope-relations for both polar regions reveal a solely bias of the glacial isotope signal on Greenland, which is caused by a strong change in the seasonal deposition of precipitation originating from nearby polar seas and the northern Atlantic. Although the performed simulations under LGM boundary conditions show a decrease of the δ18O values in precipitation in agreement with ice core measurements, the AGCM fails to reproduce the observed simultaneous decrease of the deuterium excess signal.