Pulmonary compliance (CL) and resistance (RL) were measured simultaneously in 13 supine patients prior to operation in both awake and anesthetized states. During anesthesia three air-way situations were compared: (1) mask, no airway, (2) mask, oral airway, and (3) endotracheal airway. During mask anesthesia RL. was 123 per cent above the control without oral airway, 55 per cent above the control with oral airway, and with tracheal intubation RL decreased to 18 per cent below the control. During periods of increased RL with mask, such “subtle” obstruction was clinically undetectable, not accompanied by decreased tidal exchange. The primary reason for elevated RL during mask anesthesia was due to increases in upper airway resistances. Compliance fell approximately 15 per cent during anesthesia, remained at a depressed level and was uninfluenced by the type of airway or its adequacy.