The author investigated the effect of radioactive hot spring bath on various organ cholesterol levels. As experimental subjects white male rat weighing ca. 150 g. were used. The total cholesterol of the lungs, liver, heart, kidneys and brain, and the total and ester cholesterol of the adrenals were measured by a modification of Bloor's method. The following results were obtained: 1) Single bath: The concentration of the total cholesterol of these organs and the ester cholesterol of adrenals decreased for two hours after a radioactive "Hisui-no-Yu" bath, but
such decrease was recognised only in the heart, brain, adrenals after a weak radioactive "Kenkyusho-sen". 2) A series of baths: The concentration of the total cholesterol of the lungs, heart, brain, adrenals and the ester cholesterol of adrenals decreased after a series of "Hisui-no-Yu" baths for 4 weeks. 3) A series of baths in cholesterol fed rats: The concentration of the total cholesterol of the above mentioned organs and the ester cholesterol of the adrenals in the bathing group remained decreased during the experiment comparing with the control group.