86Rb uptake of some organs and tissues, eg. both lungs, both renal cortices. small intestine, liver and skeletal muscle were studied in the control and the rabbit subjected to pneumothorax. 86Rb in the form of chloride mixed with physiological saline was intravenously. injected. The doses vere 100μc for a rabbit. The rabbits were sacrificed at intervals of 10, 20, 40, and 60 seconds after the injection of Rb, by the injection of saturated KCI solution. After sacrification, the organ and tissue sample were quickly removed. 86Rb uptake in gm of the organs and tissues were measured. On the basis of uptake value, administered doses and body weight, % dose/gm tissues per 200 gm body weight was calculated. Followings were the results: 1. Pneumothorax resulted in a marked elevation in 86Rb uptake value of collapsed lung and returned to normal level lately. 2. Contralateral lung of pnemothorax also showed marked elevation in 86Rb uptake value and recovered to normsl level. 3. Initial 86Rb uptake value of liver, small intestine of the rabbit with pneumothorax showed some elevation as compared to control, but that of late stage were similar with control. 4. Local blood flow determination by means of 86Rb uptake were inadequate in the collapsed lung of pneumothorax. 5. It was suggested that the mechanism for the initial elevation of 86Rb uptake value in each organs and tissue were different from each other.