In order to better understand the mechanisms of N(2)O emissions from nitrifying activated sludge of urban WWTPs, sludge from the Valenton plant (Paris conurbation) are subjected to lab-scale batch experiments under various conditions of oxygenation. The results show that the highest N(2)O emissions (7.1 microgN-N(2)OgSS(-1) h(-1) in average) occur at a dissolved oxygen (DO) concentration of around 1mgO(2)L(-1). These high emissions at low oxygenation (from 0.1 to 2 mg O(2)L(-1)) are due to two processes: autotrophic nitrifier denitrification and heterotrophic denitrification. Nitrifier denitrification always dominates, representing from 58% to 83% of the N(2)O production. This N(2)O production originating from nitrifying activated sludge becomes 8 times higher when nitrite is added at a DO of 1 mg O(2)L(-1); a decrease is observed both at higher and lower oxygenation. Heterotrophic denitrification represents less than 50% of the N(2)O production, decreasing from 42% to 17% when oxygenation increases from 0.1 to 2 mg O(2) L(-1). We show that ammonium oxidizing bacteria (AOB) can shift to nitrifier denitrification when oxygen is depleted in the environments including in the WWTPs, nitrite then plays the role of oxygen as the final electron acceptor. As opposed to what happens in nitrification, the end products of nitrifier denitrification are gaseous forms of nitrogen, where N(2)O is not negligible compared to N(2). Overall, N(2)O emissions represent 0.1-0.4% of oxidized NH(4)(+), depending on the oxygenation level. N(2)O emissions would range from 0.11 to 0.42 TN-N(2)O day(-1) for a tertiary treatment of the Paris wastewater effluents, consisting exclusively of activated sludge nitrification.