THAT a diminution in the oxygen tension of the inspired air can produce mental changes is an established fact. Far less certain is the possible correlation between psychotic manifestations as seen in clinical practice and an impoverished supply of oxygen to the central nervous system. It was thought that further light might be shed on this possibility by estimating the gaseous content of arterial blood and also of that drawn from the internal jugular vein of the same subject. In a group of patients in which the diagnosis of schizophrenia appears certain, this procedure has been carried out according to the method used by Lennox1, and the following results were obtained: