With the helmet and spirometer type of respiration apparatus the O consumption or the energy needs of 5 men and 1 woman were measured during periods of mental repose and mental activity. The mental effort consisted in the solution, without aid of paper and pencil, of a number of problems in multiplication and addition. To rule out the influence of excitation or "attention," the metabolism was likewise measured during periods of response to a flashing light or an electric buzzer, the subject lightly tapping a telegraph key whenever the stimulus occurred. Simultaneous graphic records were obtained of the heart rate, the respiration rate, and the ventilation of the lungs. The results of this investigation show that with intense, sustained mental effort, such as in multiplication, there is a noticeable increase in the heart rate, a rather considerable change in the character of the respiratory movements, an increase in the volume of air passing through the lungs, a small increase in CO2 production, a smaller increase in O consumption, and consequently a slight increase in the apparent respiratory quotient. The increase in O consumption, which may be taken as the best index of energy transformations, is such as to suggest that the increase in heat production as a result of intense mental effort of this type can hardly be of the order of more than 3 or 4%. In view of the sense of extreme, almost overpowering fatigue in both mind and body following sustained mental effort, it is surprising that mental effort has such an insignificant effect upon the general metabolism or level of vital activity.