Nitrogen oxides are important air pollutants which are associated with photochemical reactions in the atmosphere and adverse health effects. Although inaccuracies recently uncovered with the widely used colorimetric method invalidated much of the historical data for NO2, relatively good measurement techniques are now available. Nitrogen oxides follow a regular diurnal pattern in urban areas reflecting variations in emission sources, primarily from motor vehicles, and meteorological conditions which facilitate photochemical reactions. Although the toxicity of nitrogen oxides is not as well defined as that for other major air pollutants, evidence is accumulating that short-term exposures have different effects than the same dose administered over a longer period of time. The most important effects of NO2 appear to be on resistance to respiratory infection, whereas atmospheric nitrates seem to effect respiratory allergy.