Since cross-isentropic mass fluxes are directly related to net diabatic heating, mass fluxes into the ''overworld'' across the 385 K and 444 K isentropes are calculated using a comprehensive radiative transfer code. Ozone, methane, water vapor, and nitrous oxide data from instruments aboard the Upper Atmospheric Research Satellite (UARS) and temperature data from United Kingdom Meteorological Office (UKMO) are used for this study. Water vapor budget for the stratosphere is calculated as an independent check on the validity of the calculated mass exchange rate. The total upward mass flux is found to be strongest in northern winter and weakest in northern summer, with an annual mean flux of 73 and 119¿108 kg/s across the 444 K and 385 K isentropes, respectively, giving a turnover time of 1.6 years for the overworld. ¿ American Geophysical Union 1996