Two studies of the effect of changing CH4 fluxes on global tropospheric oxidant levels, O3, OH, H2O2, have been performed with a multi-box photochemical model. 1) A sensitivity study is made by scaling back CH4, CO and NO emissions relative to present-day budgets. 2) specific scenarios for CH4/CO/NO are selected to represent sources for the PIH and LGM. The CH4 budget is taken from an evalution of wetlands and other natural sources. For CO and NO, apparent O3 levels and ice-core-derived H2O2 for the PIH are used to constrain PIH CO and NO fluxes. There is consensus that OH has decreased since the Last Glacial Maximum, in contrast to projections for future OH, on which models are in disagreement. -from Authors