Air pollution by nitrogen oxides is a phenomenon which has been progressing rapidly in recent years. Of particular concern are the cases of pollution by nitric oxide (NO) whose concentration is frequently observed to exceed the concentration of pollution by nitrogen dioxide (NO2). As far as NO2 is concerned, environmental standards have already been established on the basis of animal experiment data and epidemiological findings which revealed negative effects caused in the respiratory apparatus by low concentrations. In contrast, the establishment of environmental standard levels with respect to NO has been left neglected due to the absence so far of data regarding its effects at low concentrations. The report presents a general examination of the toxicity of NO. The authors exposed mice for periods of up to 6.5 months to a concentration of 10 ppm, a level considerably lower than the several hundred ppm exposure concentrations previously reported and, at the same time, below the allowable concentration stipulated by the ACGIH for the industrial environment.