Pulmonary gas exchange during conventional mechanical ventilation (CMV) (tidal volume 10 ml/kg, rate 8–10 breaths/min) was compared with that during high-frequency small-volume ventilation (HFV) in 67 patients undergoing anesthesia for various surgical procedures. HFV was studied at oscillation frequencies ranging from 3 to 18 Hz with stroke volumes of 0.8 to 2.2 ml/kg. Adequate pulmonary gas exchange was achieved with CMV and HFV, and the efficiency of oxygenation, that is, (A-a)DO2, was similar in the two conditions. During HFV, the lung volume was higher than during CMV in most patients. Muscle paralysis did not significantly change either PaCO2, or PaO2. In general, increasing fresh gas flow into the HFV system above approximately 10 1/min resulted in little reduction in PaCO2, but reduction of fresh gas flow below approximately 6 1/min increased PaCO2progressively. Currently, we do not recommend HFV at 12–18 Hz for routine use during anesthesia for orthopedic or abdominal surgery.