A clear negative trend is found in the nitrogen isotopic ratios (¹⁵N/¹⁴N) of nitrate over the industrial period, based on a 100-meter ice core from Summit, Greenland. This record indicates that isotopic composition of ice core nitrate reflects changes in nitrogen oxide (NOx) source emissions and that post-industrial emissions of NO_x have resulted in a 12 (vs. air N₂) decline in δ¹⁵N of atmospheric nitrate over the last ~300 years. Interestingly, over the last glacial period (as recorded in the Greenland GISP2 ice core), the δ¹⁵N of nitrate changes by ~20 from a typical pre-industrial Holocene value of 10 to a mean glacial value of 28 , despite the lack of a significant change in nitrate concentration. The more recent ice core record clearly indicates an influence of NO_x emission sources, therefore suggesting that the glacial-interglacial change in δ¹⁵N may be a record of significant variations in the contribution of NO_x sources, such as lightning, biomass burning, biogenic soil emissions and oxidation of nitrous oxide in the stratosphere. In contrast to the source changes recorded by the nitrogen isotopes, the oxygen isotopic record of atmospheric nitrate (¹⁸O/¹⁶O, ¹⁷O/¹⁶O) in ice cores has implications for reconstruction of past atmospheric oxidant levels. This is because the oxygen isotopic composition of nitrate reflects oxidation of NO_x to nitrate in the atmosphere by oxidants such as ozone and hydroxy and peroxy radicals. Measurements of variations in the isotopic composition of atmospheric nitrate inferred from ice cores could be used to improve interpretation of other records of δ¹⁵N such as those from tree rings and/or sediments.