Abstract-The reversal of water acidification has been recently indicated by both empirical data and experiments. The world's largest whole-ecosystem "experiment" involving -30% and -40% reduction in nitrogen and sulfur emissions, respectively, has occurred in Central Europe due to the political and economic changes in the postcommunist countries since 1989. Parallel decreases in deposition rates of SO,*-, NO,-, and NH,+ have resulted in ,a rapid reversal in hydrochemistry of acidified lakes in the Sumava Mountains and the High Tatra Mountains. Concentrations of SO,>-and NO,-in lakes were reduced by 11-14 and 13-32 mmol m-', respectively, between the late 1980s and middle 1990s. Leaching of calcium, magnesium, and aluminum from the watersheds decreased while lake water pH and alkalinity increased. The immediate decline in NO,-concentrations after reduced nitrogen emissions has suggested a rapid reversibility of nitrogen saturation of the mountainous ecosystems in response to decreased amounts of nitrogen deposition.In the last two decades, studies have focused on the level of freshwater acidification in response to two opposing processes: reduction of acid deposition, and nitrogen saturation of terrestrial ecosystems. Reversibility of soil and water acidification has been indicated by both empirical data and experiments. Decreases in sulfur emission in North America and Europe, as well as experimentally reduced acid deposition, have initiated reversal of acidification and the recovery of affected ecosystems (Schindler 1988;Wright and Hauhs 1991). In contrast to the reported decreasing trends in SOd2-concentrations in acidified lakes, levels of NO,-have largely increased in the last two decades (e.g. Henriksen and Brakke 1988;Murdoch and Stoddard 1992). The reason for this is increased leaching of NO,-from N-saturated watersheds exposed to chronically high atmospheric deposition of N (Stoddard 1994;Dise and Wright 1995). NO,-outputs from the N-saturated watersheds have been thus far decreased only in large-scale experiments with drastically reduced loads of SO, 2-, NO,-, and NH, ' (Wright et al. 1988). Because the level of N emissions over the Northern Hemisphere was relatively stable in the last decade, no empirical evidence exists on the response of NO,-leaching from N-saturated terrestrial ecosystems to reduced N deposition.We herein integrate results from published and original data in order to evaluate (1) historical changes in acidifica-$on of lakes in two Central European mountain ranges (the Sumava Mountains and the High Tatra Mountains) in response to the trends in Central European emissions of S and N pollutants, and (2) empirical data on reversibility of acidification of the lakes after economically derived reduction in S and N emissions in former East Germany, Poland; Czech Republic, and Slovakia in the early 1990s. Our data represent the first observations of an immediate response of the N-saturated sites to the nonexperimental reduction in the atmospheric deposition of NO,-and NH,'.The mountain lakes...