Combination of ultraviolet (UV) disinfection and chlorination became a common multi-barrier approach taken by water utilities against microbial pathogens in drinking water disinfection. Halogenated disinfection by-products (DBPs) have been monitored and regulated in drinking water under various jurisdictions around the world for decades. More recently, several nitrogenous DBPs (N-DBPs), like haloacetonitriles (HANs) and halo-nitromethanes (HNMs), have been identified in chlorinated drinking waters, whose precursors are N-containing organic compounds of various origins and structures. Rising concerns over the N-DBPs formation due to their more potent genotoxic and cytotoxic activity than the regulated carbonaceous-DBPs have prompted extensive research on drinking water disinfection processes. To examine the effect of ammonia nitrogen (NH3-N) on N-DBPs formation during UV disinfection of drinking water followed by chlorination/chloramination, as a first attempt, synthetic waters prepared by humic acid with different NH3-N concentrations were exposed to UV dose up to 300 mJ/cm(2) from either low pressure (LP) or medium pressure (MP) lamps, followed by chlorination/chloramination. To investigate trends, the study employed synthetic waters with elevated precursor concentrations and disinfectant exposures to generate significant byproduct formation possible. LP UV treatment had no obvious effect on N-DBP formation, whereas MP UV irradiation produced more dichloroacetonitrile (DCAN). The existence of ammonia (<= 1 mg/L-N) increased DCAN concentration in high organics synthetic waters under varying MP UV doses, respectively. In high organics synthetic water samples exposed to a MP UV dose of 300 mJ/cm(2) DCAN reached a maximum yield of 13.7 mu g/L in the presence of 0.5 mg/L NH3-N, whereas lower yields were observed at NH3-N >= 2 mg/L, i.e. 2.9 mu g/L and 1.8 mu g/L at 2 and 10 mg/L NH3-N, respectively.