Nitrates minerals from the Dry Valleys of Antarctica have been analyzed for their oxygen and nitrogen isotopic compositions. The 15N was depleted with δ15N values ranging from -9.5 to -26.2?, whereas the 17O and 18O isotopes were highly enriched (with excess 17O) with δ18O values spanning 62--76? and Δ17O values from 28.9 to 32.7?. These are the largest 17O enrichments observed in any known mineral. The oxygen isotopes indicate that nitrate is from a combination of tropospheric transport of photochemically produced HNO3 and HNO3 formed in the stratosphere.