Chemical pathways involved in the nocturnal production of hydroxyl radical (OH) and associated peroxy radicals (HO2 and RO2) are quantified for various environmental situations of the continental troposphere by means of numerical simulations. In the nocturnal stable layer, most of the OH radicals are directly produced by VOC+O3 reactions in rural and semipolluted environments while in urban environments, they result mainly from the radical chain propagation from RO2 and HO2. The radical propagation is mainly driven by NO: the nitrate radical (NO3) plays no role in such processes but is significantly involved in the direct formation of RO2 radicals. Above the nocturnal stable layer, whatever the environmental situation, OH radicals are mainly due to radical chain propagation in which NO3 plays a significant role. The nighttime simulated OH concentrations are 3¿104 to 5¿105 and 1¿104 to 5¿104 molec cm-3 for the nocturnal stable and residual layers respectively.¿ 1997 American Geophysical Union