Pulmonary nitrogen clearance was studied in 16 healthy adult volunteers in the supine and two lateral decubitus positions while the subjects were 1) awake and unpremedicated, and 2) anesthetized, paralyzed, and mechanically ventilated. Five subjects were studied while they were anesthetized, breathing spontaneously. Results of the study indicate that distribution of ventilation in the anesthetized, paralyzed, and mechanically ventilated individual differs from that in the awake person and is probably more even. The slope of the alveolar nitrogen plateau and the magnitude of the cardiogenic oscillations therein were significantly decreased in the paralyzed and ventilated subjects. Functional residual capacity was significantly reduced after induction of anesthesia. Dynamic compliance of the total respiratory system was low in anesthetized, paralyzed, and ventilated subjects.