As reported from marine systems, we found that also in 15 prealpine lakes N2O concentrations were strongly correlated with O2 concentrations. In oxic waters below the mixed surface layer, N2O concentrations usually increased with decreasing O2 concentrations. N2O is produced in oxic epilimnia, in oxic hypolimnia and at oxic-anoxic boundaries, either in the water or at the sediment-water interface. It is consumed, however, in completely anoxic layers. Anoxic water layers were therefore N2O undersaturated. All studied lakes were sources for atmospheric N2O, including those with anoxic, N2O undersaturated hypolimnia. However, compared to agriculture, lakes seem not to contribute significantly to atmospheric N2O emissions.