Measurements were made of NO and NO2, in controlled experiments to investigate their production from snow. Throughout a diurnal cycle, measurements were made of ambient air and air from inside a snowblock. Enhanced concentrations of NO and NO2 (up to 15 pptv and 32 pptv respectively) were measured inside the snowblock. The production rate inside the block varied with intensity of incident radiation, and reached a maximum of 1.1¿106 molecs/cm3/s for NO and 2.1¿106 molecs/cm3/s for NO2. A second experiment, in which the snowblock was alternately exposed to sunlight and then shaded, confirmed that the diurnal production was driven by photochemistry rather than some other diurnally varying factor. Concentrations of nitrate in the snowblock did not change as a result of 50 hours of experiments, confirming that if nitrate is the source reservoir, it can not be rapidly depleted. Snowpack production may contribute significantly to NOx concentrations in the Antarctic lower troposphere. ¿ 2000 American Geophysical Union