In order to clarify the role of oxygen in the reaction of amines with nitric oxide, secondary amines were allowed to react with nitric oxide in the presence of oxygen. Although N-nitrosamines were obtained as the main products in every case, the yields depended on the substituents and reaction solvents. Detailed investigation revealed that the rection proceeded by at least two pathways : one involving oxygen as a catalyst, and the other consuming the stoichiometric amount of oxygen. Both paths afforded the same nitroso adducts. It was suggested that a third path, a catalytic process via Drago's salts was also possible. The same reaction was applied to a tertiary amine, and it was found that the oxygen was consumed stoichiometrically in this case.