Zonally averaged transport of mass and ozone into high northern latitudes exhibits a pronounced annual cycle. During summer, downwelling is at a minimum, and horizontal bulk advection and eddy fluxes of ozone and mass are found to be significant. This study examines in detail the ozone budget between 58 and 67 hPa in a meridional annulus ranging from 60¿ to 70 ¿N during summer. The area of the study was chosen to correspond with the Photochemistry of Ozone Loss in the Arctic Region in Summer high-altitude aircraft experiment based in Fairbanks, Alaska, from April to September, 1997. It is found that the seasonal cycle in transport is an important contributor to the resultant seasonal march of ozone at high latitudes. The horizontal eddy contribution to the ozone tendency at high latitudes in the lower stratosphere is found to be large and negative (reducing ozone) during middle and late summer, where it more than offsets the increasing ozone tendency due to advection by the residual circulation. During early summer, the high-latitude air mass appears to be relatively isolated, with in situ chemical processes driving the ozone tendency. ¿ 1999 American Geophysical Union