Ozone photochemistry over Antarctica in summer has been examined using a 1-D model. With diffusive transport alone, the predicted ozone decay rates from the December (summer) solstice to the March (autumn) equinox were found greatly to exceed observed rates. In order to obtain broad agreement between calculated and observed ozone amounts, it was necessary to include the effects of a slow downward mean velocity. The velocities used, a few tens of metres per day. are compatible with a Lagrangian mean circulation pattern derived from thermodynamic considerations and from GCMs.The rates of change of ozone amounts computed for 75"s decrease abruptly following the change from continuous to interrupted photolysis when the sun first sets (in mid-February) after the long polar day of midsummer. In the polar day regime, N , 0 5 concentrations are depressed strongly, and C10N02 concentrations moderately, relative to those in regions subjected to diurnal interruption of photolysis. The effects on the diurnal variations of ozone and other minor constituents are described.