In order to test the quickness of the members of the Hospital Corps Detachment, and to determine whether the changes in diet affected in any way their ability to respond promptly to sensory stimulation, each man was carried through a series of reaction experiments at the Yale Psychological Laboratory. The method of the experiments was that regularly employed in simple reaction time experiments. The person whose reaction time is to be measured is seated in a comfortable position with his finger pressing on an electric key. He is told that he is to lift his finger from the key as quickly as possible when he hears a given sound-signal. The sounder which produces this signal and the electric key are placed in a circuit with a standard time-measuring apparatus—the Hipp Chronoscope. This chronoscope is arranged so that it begins to record the instant the sound is given and stops the instant the reactor lifts his finger. The dial of the chronoscope shows in thousandths of a second (hereafter designated by the technical term _sigmas_) the time that elapses between the sound to which the reactor is to respond and the movement of response. The chronoscope was tested at frequent intervals by means of a standard pendulum and errors in the record are well under two sigmas. The time which is measured by this method is occupied chiefly by nervous processes. The following factors may be specified: First, the auditory organ is aroused by the sound; second, the afferent nerve transmits the stimulation to the central nervous system; third, the central nervous system carries the energy to the motor nerves; fourth, the efferent motor nerves transmit the stimulus to the muscles; and finally, some time is required by the muscle for its contraction. It will be seen, accordingly, that the chief factors of a reaction are nervous processes, and since the external conditions of successive reactions are in all respects uniform, any variations in the time of a given person’s reactions may be regarded as indicating variations in the nervous condition of the reactor. In view of the instability of nervous conditions, it is necessary to eliminate any slight or merely temporary fluctuations by taking each time a given reactor is tested a series of reactions. For this reason, a series of ten reactions was taken with each of the men every two weeks during November, December, and January. After an interval of two months, namely, on March 30, 31, and April 1, two final series were made with each man.