Annual and daily levels of the two major oxides of nitrogen (NOX), nitric oxide (NO) and nitrogen dioxide (NO2) are still rising in the atmosphere of the urban environment and in close proximity to automobile and airport traffic. Although oxides of nitrogen give rise to other phytotoxic pollutants such as ozone they are also in their own right damaging to plant health. This injury is not normally visible but is reflected in poorer growth and loss of productivity in terms of value and amenity. Few if any reviews have considered the effects of atmospheric concentrations of NOX as separate phytotoxic agents especially at the metabolic level. This paper is an attempt to remedy this deficiency and to promote an understanding of the problem.