The effects of anesthetic doses of ketamine (iv bolus of 3 mg · kg−1followed by a continuous infusion of 20 μg · kg−1· min−1) on functional residual capacity (FRC) measured by the helium dilution method and on the breathing pattern recorded by a noninvasive method (NIM) based on chest wall circumference changes were studied in 14 ASA P.S.I patients. Ketamine anesthesia was associated with: 1) the maintenance of FRC, minute ventilation, and tidal volume; 2) an increase in rib cage contribution to tidal breathing; and 3) an alteration of volume-motion relationships of the chest wall compartments. It is concluded that: 1) in contrast to volatile anesthetic agents, ketamine anesthesia has a sparing effect on intercostal muscle activity, which may explain the maintenance of FRC; and 2) changes in chest wall geometry and compliance induced by anesthetic agents must be taken into account for NIM to be valid.