1. Previously described methods of inhalation anæsthesia and hypoxia in psychiatric conditions have been reviewed.2. A technique of nitrous oxide administration beyond the state of intoxication and avoiding the Stage of deep hypoxemia has been described.3. Physiological changes with this form of anæsthesia are slight and the method may be considered safe.4. After the anæsthesia, a period of facilitation of mental processes is observed, which is followed by a period of relaxation and sedation.5. During the phase of facilitation, the patient has a feeling of increased well-being and shows greater spontaneity, less resistance, and enhanced insight, often expressing previously repressed ideational content and affects.6. The period of sedation lasts for several hours and is not associated with drowsiness or confusion. Sleep is usually improved the following night and other sedation is often unnecessary.7. In states of acute manic excitement, repeated daily administration of nitrous oxide has controlled and terminated the attack in some cases.8. Nitrous oxide hypoxia produces some of the features of the "shock treatments" as well as of narcoanalysis, but it is safe, convenient, and simple in administration and may be used in ambulatory patients without interruption of their ordinary activities.9. The theories on the pharmacological action of nitrous oxide and the reason for its choice are discussed.Abstract Teaser