Arylamine N-acetyltransferases (NAT) are xenobiotic-metabolizing enzymes responsible for the acetylation of many arylamine and heterocyclic amines. They therefore play an important role in the detoxification and activation of numerous drugs and carcinogens. Two closely related isoforms (NAT1 and NAT2) have been described in humans. NAT2 is present mainly in the liver and intestine, whereas NAT1 is found in a wide range of tissues. Interindividual variations in NAT genes have been shown to be a potential source of pharmacological and/or pathological susceptibility. Evidence now shows that redox conditions may also contribute to overall NAT activity. This chapter summarizes current knowledge on human NAT1 regulation by reactive oxygen and nitrogen species.