Nitrogen-enriched ground water has been proposed as an important anthropogenic source of atmospheric nitrous oxide (N₂O), yet few measurements of N₂O in large aquifer systems have been made. Concentrations of N₂O in water samples collected from the 124,000 km² central High Plains aquifer in 1999 ranged from < 1 to 940 nM, with a median concentration of 29 nM (n=123). Eighty percent of the N₂O concentrations exceeded the aqueous concentration expected from equilibration with atmospheric N₂O. Measurements of N₂O, NO₃^-, and ³H in unsaturated-zone sediments, recently recharged ground water, and older ground water indicate that concentrations of N₂O in ground water increased over time and will likely continue to increase in the future as N-enriched water recharges the aquifer. Large concentrations of O₂ and NO₃^- and small concentrations of NH₄⁺ and dissolved organic carbon in the aquifer indicate that N₂O in the central High Plains aquifer was produced primarily by nitrification. Calculations indicate that the flux of N₂0 from the central High Plains aquifer to the atmosphere from well pumping and ground-water discharge to streams was not a significant source of atmospheric N₂O.