To quantify the movement of atmospheric nitrogen deposition through two forested watersheds in the Catskill Mountains of New York, dual-isotope analysis (δ15N and δ18O) was used to differentiate NO3- derived from precipitation from NO3- derived by microbial nitrification and to quantify the contributions of these sources to NO3- in drainage waters. Samples of stream water, soil water, precipitation, snowmelt, and O-horizon soil were collected during the March and April snowmelt period of 1994 and throughout an 18-month period from August 1995 through February 1997. The mean δ18O-NO3- value of precipitation was +50.5?, whereas the mean values for stream water and soil water were +17.7? and +23.6?, respectively. The mean δ15N-NO3- of precipitation was -0.2?, that of soil water was +1.4?, and that of stream water was +2.3?; these values showed greater overlap among the three different waters than did the δ18O-NO3- values, indicating that δ15N-NO3- was not as useful for source separation. Soil water δ18O-NO3- values decreased, and δ15N-NO3- values increased, from the O to the B and C horizons, but most of the differences among horizons were not statistically significant. Nitrate derived by nitrification in incubated soil samples had a wide range of δ15N-NO3- values, from +1.5? to +16.1?, whereas δ18O-NO3- values ranged more narrowly, from +13.2? to +16.0?. Values of δ18O-NO3- indicated that NO3- in stream water is mainly derived from nitrification. Only during a high-flow event that exceeded the annual flood was precipitation a major contributor to stream water NO3-. Values of δ18O-NO3- and δ15N-NO3- changed at differing rates as NO3- cycled through these watersheds because δ18O-NO3- values change sharply through the incorporation of oxygen from ambient water and gas during nitrification, whereas δ15N-NO3- values change only incrementally through fractionation during biocycling processes. The results of this study show that most NO3- is first cycled through the biota and nitrified before entering the stream.