Δ15N of N2O and NO-3 were measured to determine the mechanisms producing N2O in nitrate contaminated groundwaters. These groundwaters contained N2O at concentrations between thirty to one hundred times higher than the atmospheric equilibrium. The N2O/NO-3 molar ratios in groundwaters were from 0.2¿10-3 to 4.4¿10-3. The Δ15N avlues of N2O, -5.8 to -21.3 permil, were much lower than those of coexistent NO-3, +3.6 to +7.3 permil. In the light of evidence that 15N in N2O from nitrification is highly depleted, the present findings suggest nitrification as a dominant mechanism for the production of N2O in the groundwaters. However, unlike other aquatic systems where nitrification is a major route of N2O production, no clear correlation was observed between ΔN2O and -ΔO2 in the groundwater. This poor correlation strongly suggests that N2O in the groundwater was predominantly produced in the soil column and/or unsaturated zone. Based on two-year observation of N2O flux from a spring, a possible charging rate of N2O to the aquifer was estimated as 1.2--2.3 mg N2O-N⋅m-2⋅y-1. The rate is much smaller than the reported soil emission rates, suggesting the groundwater N2O to be an insignificant source for atmospheric N2O. Large discrepancies in Δ15N of N2O between the groundwaters and the surface airs supports this view. ¿ American Geophysical Union 1991