The interannual variability and trend in the CH4 lifetime, as a measure for global mean OH concentration, have been analyzed systematically with three-dimensional (3-D) chemistry-transport model simulations. It is shown that the global mean OH concentration is highly variable from year to year due to changes in meteorology, changes in tropospheric UV radiation intensities, and changes in chemical concentrations owing to variable emissions of photochemical precursor gases (CH4-CO-NMVOC-NOx). The meteorological variability is taken into account with the ECMWF-ERA15 1979--1993 reanalysis. Satellite observations provide the observed changes in the stratospheric ozone concentrations. Emission inventories are used to account for trends in anthropogenic emissions and their patterns. For the period 1979--1993, our simulations indicate a decrease of the calculated global tropospheric methane lifetime from 9.2 to 8.9 years, corresponding to a positive OH trend of 0.24 ¿ 0.06% yr-1. The modeled trend is mostly determined by changes in the tropical tropospheric water vapor content, while the changes in photolysis rates and in surface emissions of reactive trace gases compensate in their effect on the calculated OH trend over the analyzed time period.