In an attempt to further clarify the effect of X-irradiation on the activity of surfactant in rabbits, X-ray in dose of 900r was irradiated to the lung tissues of rabbits in vitro. Tension-area diagram of the lung extract was recorded automatically by a modified Langmuir-Wilhelmy balance with a synchronized recording system designed in this department. The surface tension of the 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 thus obtained are summarized as follows: I The maximal surface tension, minimal surface tension, width of the tension-area diagram at the surface area of 40% in the lung extract and stability index of the normal rabbit long extract were 40.73 dynes/cm, 8.96 dynes/cm, 20.71 dynes/cm and 1.28, respectively. II. When 900r of X-ray was irradiated to the lung in vitro, 1) The maximal and minimal surface tensions did not differ noticeably from the normal at 1,3, and 5 post-irradiation hours, but the minimal surface tension increased significantly at 24 and 48 hours Post-irradiation. 2) The width of the tension area at the surface area of 40% showed a tendency of decrease throughout the experiment. 3) The stability index showed no significant change at 1,3 and 5 post-irradiation hours,but at 24 and 48 hours post-irradiation a significant decrease was observed comparing with the control. III. Activity of surfactant was significantly depressed by X-irradiation in vitro especially at 24 and 48 hours post-irradiation.