Conflicting results have been reported regarding the effects of ether anesthesia on the surfactant system of the lung, and little has been known on the effects of methoxyflurane anesthesia on the activity of the pulmonary surfactant. In the present study, an effort was made to observe and clarify possible effects of ether and methoxyflurane anesthesia on the activity of surfactant of the rabbit lung extracts, subjecting the rabbit to the light or deep anesthesia with the anesthetics by the non-rebreathing system. Tension-area (T-A) diagram of the lung extract was recorded automatically by a modified Langmuir-Wilhelmy balance with a synchronized recording system. The arterial blood pressure and heart rate during the entire stages of anesthesia were also recorded. The surface tension of the lung extracts, arterial blood pressure and heart rate thus obtained were compared with those of the non-anesthetized control group, and the results are summarized as follows: 1, The maximal surface tension, minimal surface tension, width of the T-A diagram at the surface area of 40% and stability index of the control group were 35.15±4.49 dynes/cm, 12.40±2.73 dynes/cm, 18.15±4.0 dynes/cm and 0.96±0.23, respectively.2. No significant difference was observed in the activity of surfactant of the lung extract prepared from the rabbit anesthetized with ether or methoxyflurane throughout the entire experiment. 3. In the ether anesthesia group, arterial blood pressure decreased gradually from the control value of 102.38±10.99 mmHg as the anesthesia deepened. The heart rate was slightly elevated in light stages of ether anesthesia from the control value of 284.95±35.91/min, but 120 min after deep anesthesia the heart rate returned to the control value. 4. Anesthesia with methoxyflurane produced a significantly decreased arterial blood pressure from the beginning and throughout the entire anesthesia stage. The heart rate did not change significantly througbout the entire anesthesia compripg with the control. 5. The above results suggest that the inhalation anesthesia with ether or methoxyflurane do not produce any significant change in the activity of pulmonary surfactant.