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A photochemical model is used to predict temporal trends in OH, CH4, and CO over the next 50 years and to assess possible past changes in these trace gases from 1860 to 1985. Various scenarios of perturbed CH4 and CO levels based on recently reported CH4 and CO data are simulated at several levels of background NOx. With low NOx conditions (NO + NO2 = 25 pptv (parts per trillion by volume)), typical of the nonpolluted troposphere, we compute a monotonic loss of tropospheric OH from 1860 to 2035, with the magnitude of the decrease dependent on CO and CH4 increases during the period. If current trends continue (ground-level mixing ratios (mole fractions) of CH4 rising about 1% per year), by 2035 northern latitude CH4 will increase from 1.6 to 2.9 ppmv (parts per million by volume) and CO will double or triple its present day level (to about 250–350 ppbv (parts per billion by volume) in the nonpolluted northern hemisphere). The column abundance of OH in the background troposphere will decrease 25–35%, depending on the magnitude of CH4 and CO increases and assuming that global temperature increases do not raise water vapor levels during that time. Calculations with increased H2O show a 17–30% decrease in OH. Under higher-NOx conditions (1 ppbv), OH shows a decline from 1860 to 2035, but it shows only half as much decline as with low-NOx conditions. In the case of NOx in a transitional zone (i.e., NOx increasing from 20 pptv in 1860 to 0.5 ppbv in 2035), CO and CH4 increases accompany a rise in OH, followed by a small decline. The turning point in OH depends on the rate of change in NOx and O3. Recently observed upward trends in CO and CH4 are probably due to increasing emissions of both CH4 and CO. We always compute a temporal increase in tropospheric O3 when CH4 and CO increase. Typical 1860 values for surface O3 are 25 ppbv, compared to 30 ppbv (NOx = 25 pptv)and 40 ppbv (NOx = 1 ppbv) in 1985. In a transition NOx zone we find surface O3 increasing from 10 ppbv in 1860 to 27 ppbv in 1985.