Apnea and desaturation following nitrous oxide inhalation were studied in seven adult volunteers breathing spontaneously. Arterial oxygen saturation (SpO2), end-tidal CO2 concentration in the nasal cavity and respiratory patterns were measured in volunteers breathing air after N2O (50% or 67%) + O2. SpO2 was measured with Biox 3700 and end-tidal CO2 concentration was measured with Normocap, and respiratory patterns were recorded with RESPIGRAPH. After breathing N2O, two volunteers had frequent apnea (greater than 20 sec) accompanied by desaturation (SpO2 less than 90%). The lowest value of SpO2 was 82%. When the apnea occurred, the airway seemed to be open and end-tidal CO2 concentration values were lower than those before N2O inhalation. The authors considered that this kind of apnea was due to several factors, such as hypocapnia caused by hyperventilation during N2O anesthesia, dilution of alveolar O2 and CO2 during N2O excretion, loss of consciousness by N2O, and depression of CO2 ventilatory response by N2O. Inhalation of O2 at high concentrations for five minutes could improve the hypocapnia and prevent the apnea.