Nitrogen and oxygen isotopes of NO3- have been measured in snow and firn from Summit, Greenland. The 15N/14N and 18O/16O ratios of NO3- in recently fallen snow are similar to those of surface snow. Diurnal variation is observed in 15N/14N of NO3-, and possibly 18O/16O, suggesting fractionating loss of NO3- from snow during the day, which is subsequently recovered at night. A larger seasonal variation is observed, with higher 15N/14N and lower 18O/16O of NO3- in summer than winter, which cannot be explained by postdepositional fractionation. The generally high 18O/16O of NO3- in Greenland snow (δ18O versus VSMOW = 65.2 to 79.6?) indicates that oxygen atoms from ozone have been incorporated into NOx that was subsequently deposited as HNO3. The lower mean δ18O of NO3- in summer snow relative to winter (68.9? in summer 2000 and 70.5? in summer 2001 versus 77.5? in winter 2000--01) is a result of summertime HNO3 production via NO2 reaction with hydroxyl radical (OH), which dilutes the high δ18O imparted on NO2 from ozone. The higher mean 15N/14N of NO3- observed in snow from spring (δ15N versus air N2 = +5.9? in 2000 and -1.4? in 2001) and summer (+0.1? in 2000 and -0.8? in 2001) than fall (-9.2? in 2000) and winter (-10.0? in 2000--01) is more difficult to explain with seasonal photochemistry, given current knowledge. The seasonal 15N/14N change may reflect NOx sources, with a greater fall and wintertime contribution from fossil fuel emissions relative to other inputs of NOx (i.e., biogenic soil emissions, biomass burning, and lightning).