Possible ways in which disturbances of the nitrogen cycle might result in deleterious effects on human and animal life are discussed, including the hazardous properties of oxides of N, nitrate and nitrite, and certain N-nitroso compounds that may be found in the environment. The main biological actions of oxides of N are caused by NO<latex>$_2$</latex>, which is a powerful respiratory irritant. Nitrates have relatively low toxicity but nitrites can cause methaemoglobinaemia, which may be fatal, particularly in infants. Nitrosamines occur in the environment in very low concentrations in certain foods, in tobacco smoke, and in the atmosphere in some areas. Carcinogenic nitrosamines can also be formed in the body by reaction of endogenous amines with nitrites, part of the latter being derived from saliva. Although nitrosamines are established as powerful carcinogens in animals, their possible role in human cancer is uncertain and requires further study.