General anesthesia results in increases in respiratory elastance and flow resistance within 10-15 min after induction. Stabilization (compensation) of the respiratory system in the face of this added load is related to the addition of active (contractile) force-length and force-velocity properties to its internal impedance during inspiration. The difference between active (inspiratory) and passive (relaxation-exhalation) values of elastance and resistance can be used as an index of load compensation. We therefore evaluated the effects of duration of anesthesia on respiratory impedance and stabilization by comparing active elastance (E'rs) and flow resistance (R'rs) to their corresponding passive values (Ers, Rrs) at the beginning and end of steady-state breathing in ten young, healthy anesthetized adults undergoing orthopedic surgery (anesthesia approximately 1.5 MAC of a halogenated anesthetic in 60% N2O-40% O2). Occlusion pressure (P0.1) and components of ventilation also were measured. Duration of anesthesia did not correlate with changes in active or passive mechanics or with control of ventilation. Mean Rrs increased by 76% (P less than 0.025), probably due to a decline of atropine effect; however, R'rs increased by only 17%, indicating near-maximum stabilization of flow-resistive properties at the end. Passive elastance increased 6%, whereas E'rs increased 3.8%, indicating essentially constant volume-elastic stabilization throughout. Occlusion pressure increased 20% and VT/TI 22%, probably due to a decline in effects of sedation and neuromuscular blockade. We conclude that after induction of anesthesia, the reserve available to overcome flow resistance (intrinsic plus equipment) diminishes but is not related to duration of anesthesia. The reserve available to overcome elastic properties remains essentially constant throughout anesthesia.