Motokawa and Suzuki reported a new method for measuring fatigue, in which the main procedure is as follows: Repetitive rectangular pulses of electric current of 20 cycles per sec. are applied for stimulation of the subject's eye in such a manner that the voltage is increased at a constant rate from zero to such a level as the subject perceives distinct flickering phosphenes, and then decreased at the same rate until he feels no flicker. Readings of the stimulating voltages are taken at moments, when the subject gives signals informing the appearence and disappearance of the flickers, in order to obtain both sorts of threshold for appearance (S1) and disappearance (S2). Motokawa and Suzuki proposed to use the difference (ΔS) between these two threshold values, i. e. S1-S2 as a measure of fatigue. By using this method we measured the value of ΔS of the bathed people and researched the action of hot spring bathing on living body and obtained the the following results. 1) This experiment was applied to 5 healthy adult men who bathed for 10 minutes in Yuze hot spring of 43°C. The value of ΔS decreases temporarily immediately after bathing and after it increases fairly rapidly and after about between an hour and two it returnes to the value before bathing, and then it becomes lower than the value before bathing. If we regard tile increasing of the value of ΔS as a phenomenon of fatigue, the decreasing of the value of ΔS must be a promotion of cell function of living body, that is, activation of protoplasm. It is thought that bathing in this hot spring may be helpful for recovery of fatigue. 2) When these men bathed in state of fatigue, after bathing the value of Δs decreases strikingly. That must be, the bathing influences for recovery of fatigue. 3) The body temperature increases after a meal as well as after bathing. But the value of ΔS increases after a meal, while it decreases after bathing for a while. By these facts we can conclude that there is no direct relation between the body temperature and the value of ΔS.