Since Hick suggested that the reaction time increased linearly as a function of the information transmitted, various investigations on this subject have been made. In the present paper an attempt has been made to summarize the results of our recent experiments in this field and to find some available hypotheses which may provide a solution for the problems in industrial psychology. such as fatigue, accident proneness, development of skill, etc. There are two aspects in the results of reaction time experiments; one is the lawfulness of the mean reaction time, and the other the variability of each reaction time. The former leads us to the theory of "single channel" and the theory of "psychological refractory" period, the latter shows the individual differences, the degree of individual adjustment to his environment. As for the mental fatigue, the results led us to the hypotheses that the fatigue was caused not by the lower efficiency of local responses but by the disorder in central control mechaanisms. From this viewpoint, it may be said that the "fatigue test"attempting to measure the degree of fatigue, by the scale of sensitivity of a single organ is inappropriate for its purpose.