Langtjern is an acid lake situated in south-central Norway, 516 m above sea level. The watershed is underlain by biotite gneisses and granites. Coniferous forests cover 63% of the watershed, while 16% is covered by peaty areas. The lake has a relatively large watershed (4.8 km2) in relation to lake size (0. 23 km2), and most of the water and chemicals reach the lake via the watershed and inflowing streams. Direct precipitation on the lake surface is less important. The two major inflowing streams show rapid responses to influxes of acid precipitation (weighted annull mean pH 4.3) and long recovery periods between episodes. The fall and spring acid episodes have different impacss on the lake-water chemistry. The spring episode during which large amounss of H+ are released in the first phases of the snow melt reduces the pH in only the surface layers of the still ice-eovered lake and much of the inflowing pollutants are promptly discharged. In the fall, however, the acid inputs are readily mixed through the entire water column during the autumnal circulation period. Assuming chloride as a conservative parameter, mass-balance calculations show that there is a net loss of Ca, Mg and Al from the watershed while there is a net uptake of H+, NO3 and NH4. SO4 and Na budgets balance. The net uptake of H+ in the watershed is directly related to net losses of other cations. The net uptake of NO3 and NH4 is most likely due to biological uptake by the forest ecosystem. For the Langtjern itself the inputs of all major ions equal the outputs, except for H+ and NO3 which are retained by the lake. The neutralization of H+ in Langtjern varies seasonally, whereas only 10-20% is neutralized during spring, in the autumn 50-70% is neutralized.