In situ measurements of radical and long-lived species were made in the lower Arctic stratosphere (18 to 20 km) between spring and early autumn in 1997. The measurements include O3, ClO, OH, HO2, NO, NO2, N2O, CO, and overhead O3. A photochemical box model constrained by these and other observations is used to compute the diurnally averaged destruction and production rates of O3 in this region. The rates show a strong dependence on solar exposure and ambient O3. Total destruction rates, which reach 19%/month in summer, reveal the predominant role of NOx and HOx catalytic cycles throughout the period. Production of O3 is significant only in midsummer air parcels. A comparison of observed O3 changes with destruction rates and transport effects indicates the predominant role of destruction in spring and an increased role of transport by early autumn. ¿ 2000 American Geophysical Union