Two mechanisms contribute to interhemispheric mass exchange: large scale eddy transfer and mean meridional circulations. Estimates of both are presented based on data from a recently completed study of the general circulation of the tropics. The long term interhemispheric exchange time is found to be about 320 days. It is suggested that when eddy processes reinforce the transfer by mean meridional circulations in the upper troposphere in late spring and summer much faster exchange rates may occur which may be responsible for an apparent anomaly in the seasonal variation of surface ozone in the southern hemisphere. Finally the interhemispheric mass flux is compared with a crude estimate of the tropospheric-stratospheric mass flux based on the same tropical data.