1. Twenty-three patients suffering from psychoses occurring within the senium were investigated with regard to their cerebral oxygen consumption.2. The arterial oxygen was low (16.31 vol. per cent). It is suggested that this is in part due to the emphysema and increase in residual air frequently found in aged persons; it may be due in part to failure of diffusion from the capillary alveolar walls.3. The oxygen-carrying power of the blood was reduced, as indicated by an erythrocyte count of 4,220,000 and a hemoglobin of 87 per cent (Sahli).4. The arm to carotid circulation time was increased (25.6 seconds).5. If the cerebral circulation time is reduced, as we have shown the arm to carotid circulation time to be, then the normal cerebral A-V difference in our patients would indicate a diminished oxygen consumption by the cerebral tissues.