OH and HO2 radicals are the dominant oxidants in the tropospheric environment. They are generated from O3 and H2O in the presence of sunlight and are subject to rapid cyclic interconversion driven by CO and NO. From a zero-dimensional box-model calcn. the steady-state concns. of HOx in the polluted continental and in the clean maritime troposphere at noon and in the summer are estd. to be [OH] = 1.2 * 106 and 4.3 * 105 cm-3 and [HO2] = 3.8 * 107 and 2.0 * 108 cm-3, resp. The addn. of hydrocarbons to the photooxidn. system is treated in detail for CH4 as a model compd. Due to the different oxidn. mechanism the injection of CH4 results in an increase of HOx in the polluted troposphere, whereas the opposite is the case for clean tropospheric air. For the present day CH4 mixing ratio of 1.7 ppm this effect is still small, but it leads to increased differences in oxidant content between polluted and clean tropospheric regions if CH4 and other hydrocarbon emissions continue to increase at current rates.