An average tropospheric OH concentration in the Northern Hemisphere of about 3¿105 cm-3 is not in disagreement with present photokinetic information. This has important implications for the tropospheric budgets of CH4, CO, H2 and CH3CCl3. Present concentrations of CH4 and H2 in the atmosphere may be substantially larger than those prevailing during the pre-industrial era, implying also an increase in H20 concentrations and a decrease in ozone concentrations in the upper stratosphere. Accepting this estimate of an OH concentration and an important CO sink at the ground (Seiler, 1974), there must be additional sources of CO than considered in presently evaluated budgets.