To clarify the biological processing of nitrate within temperate forested catchments using unprocessed atmospheric nitrate exported from each catchment as a tracer, we continuously monitored stream nitrate concentrations and stable isotopic compositions including 17 O-excess (Δ 17 O) in three forested catchments in Japan (KJ, IJ1, and IJ2) for more than two years. The catchments showed varying flux-weighted average nitrate concentrations: 58.4, 24.4, and 17.1 µmol L −1 in KJ, IJ1, and IJ2, respectively. In addition to stream nitrate, nitrate concentrations and stable isotopic compositions in soil water were determined for comparison in the most nitrate-enriched catchment (the KJ site). While 17 O-excess of nitrate in soil water showed significant seasonal variation, ranging from +0.1 to +5.7 ‰, stream nitrate showed small variation, from +0.8 to +2.0‰ in KJ, +0.7 to +2.8 ‰ in IJ1, and +0.4 to +2.2‰ in IJ2. We concluded that the major source of stream nitrate in each forested catchment was nitrate in groundwater, which buffered the seasonal variations in soil water nitrate. The estimated annual export flux of unprocessed atmospheric nitrate accounted for 9.4 ± 2.6 %, 6.5 ± 1.8 %, and 2.6 ± 0.6 % of the annual deposition flux of atmospheric nitrate in KJ, IJ1, and IJ2, respectively. The export flux of unprocessed atmospheric nitrate relative to the deposition flux showed a clear normal correlation with the flux-weighted average concentration of stream nitrate, indicating that reductions in the biological assimilation rates of nitrate in forested soils, rather than increased nitrification rates, are likely responsible for the enrichment of stream nitrate, probably due to nitrogen saturation. The export flux of unprocessed atmospheric nitrate relative to the deposition flux in each forest ecosystem is applicable as an index for nitrogen saturation.