Adult rabbits were anesthetized with nembutal, 30 mg/kg. Carotid artery and jugular vein were exposed surgically and cannulated with polyethylene tubing. Arterial blood pressure was recorded via pressure transducer on the physiograph and 100μg/ml of histamine solution was infused through the jugular vein by using the constant infusion pump with a rate of 0.92 ml/min or 1.40 ml/min. Mean arterial blood pressure was maintained at 40 ~ 70 mmHg and hypotension was kept for 2 hours. After the termination of this period, blood was taken and osmotic fragility was mea sured immediately. Also, every sample of normal blood and shocked blood was incubated for 1 hour or 2 hours at 37℃ in order to see whether or not there was some influence of incubation. Furthermore to clarify which component was responsible for the change on the fragility, the mixtures of normal blood cells with shocked plasma and shocked blood cells with normal plasma were also incubated at 37℃ for one or two hours and fragility in such cases was measured. The data obtained were analysed by probit-plot method and the concentration of saline solution at which the hemolysis started to occur, 50% of blood cells were hemolysed and that at which the red blood cells hemolysed completely were determined. The values for the blood of hypotension stage were compared with those of the control blood. The results obtained were as fellows: 1. Osmotic fragility of red blood cell was increased in hypotensive state induced by histamine. 2. The differences of osmotic fragility after two hours of incubation were negligible both in normal blood and in that of hypotensive state. 3. Osmotic resistance of normal red blood cell incubated in shock plasma was less than that of shock red blood cell incubated in normal plasma. It was suggested that plasma in hypotensive state caused by histamine might be primarily responsible for the alteration of red blood cell fragility.