In order to observe a possible effect of CO intoxication on the activities of surfactant, the rabbit war exposed either to 0.1, 0.2 and 0.4% CO gas for two to six hours or 0. 1% CO gas for two hours daily for 1, 3 and 5 days, and the lung extract was prepared. The tension-area diagram of the lung extract was recorded automatically using the modified Langmuir-wilhelmy balance with a synchronized recording system, and the results were compared with control (i. e. : non-CO gas exposed normal rabbits). The results obtained are summarized as follows: 1. The maximal and minimal surface tensions, 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 31.6±3.11 dynes/cm, 8.2±0.56 dynes/cm, 21.4±4.40dynes/cm and 1.12±0.22, respectively. 2. The activities of surfactant in the CO gas exposed group did not show any significant change from the control. 3. The above results suggest that CO gas produced no noticeable effect on the surfactant system of the lung.