Radon (Rn) contents in expired air after bathing, exposure in hot-air room and drinking of Misasa radioactive hot spring water were determined using an ionization chamber equiped with vibrating reed electrometer. The results were as follows; 1. Rn Contents in the indoor and outdoor air of Misasa spa were in the range of 0.5-1.0 pCi/ℓ and 0.4-0.7 pCi/ℓ. 2. Rn contents in the expired air of persons living at Misasa spa area and none spa area were 0.4-0.9 pCi/ℓ, 0.6±0.2 (S.D.) pCi/ℓ and 0.1-0.9 pCi/ℓ, 0.5±0.2 (S.D.) pCi/ℓ respectively and no difference of Rn contents was observed in both groups. 3. The highest Rn contents in the expired air of subjects bathed in radioactive hot spring (Rn ; 58.0×10(-10) Ci/kg, watre temp. : 41±1℃.) were immediately after bathing, and the values were 10.8-25.5 pCi/ℓ (bathing for 5 min.), 16.0-27.9 pCi/ℓ (bathing for 10 min.) and 38.8-59.3 pCi/ℓ (bathing for 15 min.) respectively. Rn contents in expired air were reduced to about 1.0 pCi/ℓ during 120-180 min. after bathing. The longer the bathing time and the younger the subjects, the higher Rn contents in the expired air. The biological half-life of Rn in the expired air of subjects after bathing was 42-43 min. 4. The highest Rn contents in the expired air of subjects exposed in hot-air room (Rn in air; 54.3pCi/ℓ, air temp. : 37-38℃., humidity: 40%, staying for 15 min.), and the values were 4.9-7.8 pCi/ℓ, and gradually reduced to about 1.0 pCi/ℓ during 120 min. The bioligical half-life of Rn in the expired air after leaving the hot-air room was 43 min. 5. Rn contents in the expired air of subjects immediately after drinking of radioactive spring water (Rn contents: 596.8×10(-10) Ci/kg, 500mℓ), were the highest, and the value were 28.0-101.5 pCi/ℓ, and reduced to about 1.0pCi/ℓ during 180 min. after drinking. The biological half-life of Rn in the expired air was 40 min.