The carcinogenic effect of nitroso compounds, and of nitrosamines in particular, is well-known. The possibility that these compounds form as a result of the action of nitrogen oxides on lung tissue has been assumed on the grounds that nitrosamines in tobacco smoke develop as a result of the reaction between NO2 and amino compounds, and that the amino groups of pulmonary proteins, or their decomposition products, therefore could be also nitrosylated. Serfontein (1966), Neurath (1967) and Tomatis (1967) found traces of nitrosamines in tobacco smoke, but not in lung tissue. The report is concerned with the problem of the effect of nitrogen oxides on the organism, and also looks into the possibility of the origin of nitroso compounds and nitrosamines in the exposed lungs of animals.