Samples of air at various depths in firn were collected at Vostok Station, Antarctica, and analyzed for δ15N of N2, O2/N2 ratio, and CO2. The ultimate objective of this work is to constrain the recent rate of the atmospheric [O2] decrease, thereby providing a direct experimental constraint on net CO2 fluxes into the ocean and the land biosphere. δ15N increases with depth, because of gravitational enrichment, at approximately the rate predicted by the barometric equation. Gravitationally corrected CO2 decreases with depth to 308 ppmV at 101.9 m depth, because deeper air is older and less contaminated with anthropogenic CO2. The gravitationally corrected O2/N2 ratio increases with depth mainly because burning fossil fuel consumes O2. Samples in the top 20 m of the firn have anomalously high CO2 concentrations and anomalously low O2/N2 ratios. Samples below 96.2 m depth have anomalously high O2/N2 ratios. Between 30 and 96.2 m depth, the gravitationally corrected increase in the O2/N2 ratio is nearly equal to that computed from the rate of O2 consumption by combustion of fossil fuels. Our results indicate that the rate of anthropogenic O2 consumption can be accurately constrained by future firn air studies. Copyright 1994 by the American Geophysical Union.