The impurities potentially contained in nitrous oxide cylinders include NO, NO₂, CO and NH₃. The consequences of inahaling NO and NO₂ are reflex breathing inhibition with laryngospasm and the rapid onset of intense cyanosis due to altered pulmonary gas exchange and methaemoglobinemia. Pulmonary edema may occur in the acute stage. Chronic chemical pneumonitis may follow with resultant pulmonary fibrosis and the respiratory and metabolic acidosis may also occur. Hypotension may be marked and results from the effects of nitrate and nitrite ions on vascular smooth muscle. Since certain clinical features provide clues that allow practicing anesthetists to recognize the toxicities of higher nitrogen oxides, six of our cases will be described in detail, whereas the remaining cases will be only commented. We examined a N₂O cylinder at the National Metrology Institute, its NO_X concentration was 2359 ppm. Another two cylinders were also abnormal with NO_X concentrations of 547 ppm and 180 ppm. We recommend that medical gases be subjected to quality control on a national basis.