In an attempt to clarity the effect of X-irradiation on the activity of surfactant in rabbits, and also to observe the possible difference when the irradiation was made in single or fractionated dose, X-ray in dose of 900 r was irradiated to rabbits either in single or fractionated dose of 300 r each day for three day, Tension-area diagram of lung extract was recorded automatically by a modified Langmuir-Wilhelmy balance with a synchronized recording system designed in this department. The surface tension of lung extract was measured at 1,3,5,24, and 48 hours post-irradiation, and the results were compared with the non-irradiated normal group. The result obtained are summerized as follows: 1) The maximal surface tension, minimal surface tension, width of the tension-area diagram at the surface area of 40% in lung extract, and stability index of the normal rabbit lung extracts were 40.73 dynes/cm, 8.96 dynes/cm. 20.71 dynes/cm, and 1.28 respectively. 2) Activity of surfactant was significantly depressed by X-irradiation, and the pattern of depression was more prominent in the single irradiation group than in the fractionated group. 3) It was found that the changes of the width of the tension-area diagram at the surface area of 40% in lung extract and stability indices corresponded well with that of the maximal or minimal surface tension.