We investigated the effects of the prone position on the mechanical properties (compliance and resistance) of the total respiratory system, the lung, and the chest wall, and the functional residual capacity (FRC) and gas exchange in 17 normal, anesthetized, and paralyzed patients undergoing elective surgery. We used the esophageal balloon technique together with rapid airway occlusions during constant inspiratory flow to partition the mechanics of the respiratory system into its pulmonary and chest wall components. FRC was measured by the helium dilution technique. Measurements were taken in the supine position and after 20 min in the prone position maintaining the same respiratory pattern (tidal volume 10 mL/kg, respiratory rate 14 breaths/min, FIO2 0.4). We found that the prone position did not significantly affect the respiratory system compliance (80.9 +/- 16.6 vs 75.9 +/- 13.2 mL/cm H2O) or the lung and chest wall compliance. Respiratory resistance slightly increased in the prone position (4.8 +/- 2.5 vs 5.4 +/- 2.7 cm H2O.L-1.s,P < 0.05), mainly due to the chest wall resistance (1.3 +/- 0.6 vs 1.9 +/- 0.8 cm H2O.L-1.s, P < 0.05). Both FRC and PaO2 markedly (P < 0.01) increased from the supine to the prone position (1.9 +/- 0.6 vs 2.9 +/- 0.7 L, P < 0.01, and 160 +/- 37 vs 199 +/- 16 mm Hg, P < 0.01, respectively), whereas PaCO2 was unchanged. In conclusion, the prone position during general anesthesia does not negatively affect respiratory mechanics and improves lung volumes and oxygenation.