The feasibility of using nitrogen and oxygen isotope ratios of nitrate (NO3 -) for elucidating sources and transformations of riverine nitrate was evaluated in a comparative study of 16 watersheds in the northeastern U.S.A. Stream water was sampled repeatedly at the outlets of the watersheds between January and December 1999 for determining concentrations, δ15N values, and δ18 values of riverine nitrate. In conjunction with information about land use and nitrogen fluxes, δ15Nnitrate and δ18Onitrate values provided mainly information about sources of riverine nitrate. In predominantly forested watersheds, riverine nitrate had mean concentrations of less than 0.4 mg NO3 --N L-1, δ15Nnitrate value s of less than +5‰, and δ18Onitrate values between +12 and + 19‰. This indicates that riverine nitrate was almost exclusively derived from soil nitrification processes with potentially minor nitrate contributions from atmospheric deposition in some catchments. In watersheds with significant agricultural and urban land use, concentrations of riverine nitrate were as high as 2.6 mg NO3 --N L-1 with δ15Nnitrate values between +5 and +8‰ and δ18Onitrate values generally below +15‰. Correlations between nitrate concentrations, δ15Nnitrate values, and N fluxes suggest that nitrate in waste water constituted a major and nitrate in manure a minor additional source of riverine nitrate. Atmospheric nitrate deposition or nitrate-containing fertilizers were not a significant source of riverine nitrate in watersheds with significant agricultural and urban land use. Although complementary studies indicate that in-stream denitrification was significant in all rivers, the isotopic composition of riverine nitrate sampled at the outlet of the 16 watersheds did not provide evidence for denitrification in the form of elevated δ15Nnitrate and δ18Onitrate values. Relatively low isotopic enrichment factors for nitrogen and oxygen during in-stream denitrification and continuous admixture of nitrate from the above-described sources are thought to be responsible for this finding.