Development of mutagenicity of five N-nitrosamines (N-nitrosodimethylamine (NDMA), N-nitrosodiethylamine (NDEA), N-nitrosodi-n-propylamine (NDPA), N-nitrosopyrrolidine (NPYR) and N-nitrosodiphenylamine (NDPhA)) was investigated during oxidative processes involving UV-photolysis, ozone and OH radicals. The mutagenicity was detected by the Ames test with 3 different strains, TA98, TAMix and YG7108, a strain which is sensitive for N-nitrosamines, in presence and absence of metabolic activation (S9). UV photolysis of mutagenic N-nitrosamines (NDMA, NDEA, NDPA and NPYR) leads to the removal of their specific mutagenic activity as detected in YG7108 in the presence of S9. A formation of mutagens during UV photolysis was detected only in case of NDPhA in the strain TA98. Oxidation products of NDMA, NDEA and NDPhA did not show any significant mutagenicity in the strains used, whereas oxidation of NDPA and NPYR by hydroxyl radicals seems to lead to the formation of direct mutagens (mutagenic in the absence of S9) in YG7108 and TAMix. Oxidation by hydroxyl radicals of N-nitrosamines with chains longer than ethyl can mimic metabolic activation of N-nitrosamines in vivo.