Seasonal variations of the mass flux between the stratosphere and troposphere are investigated using a Lagrangian framework. Monthly three-dimensional trajectories are calculated for 1996 and 1997 from the United Kingdom Meteorological Office assimilated data. Mass exchange shows nearly the same annual variations as previous Eulerian studies: (1) the northern net downward flux across the extratropical tropopause shows a primary maximum in late spring and early summer and secondary in winter, which is consistent with observational measurements, (2) the southern net downward flux has a peak only in winter, and (3) the extratropical and tropical air exchange across the upper boundary of the middleworld exhibits a pronounced annual variation with a wintertime maximum and summertime minimum. The occurrence of the largest downward flux over the western edges of the Pacific and Atlantic and near the Mediterranean suggests that tropopause folding and baroclinic disturbances in midlatitude storm tracks are the primary agent for the intrusion of stratospheric air into the troposphere. Tropical upward transport occurs in association with the monsoon anticyclones, and the greatest diabatic injection into the tropical stratosphere takes place over the summer hemisphere subtropics according to the seasonal variation of the Hadley circulation. Comparison of the Lagrangian flux estimate across the 100-hPa surface with the estimate by the downward control mechanism using the residual mean meridional circulation shows reasonable correspondence.