This study was designed to determine the responses of lung volume and respiratory resistance (Rrs) to decreasing levels of continuous negative airway pressure (CNAP). Twenty normal subjects were studied in the basal state and under CNAP levels of 5, 10, and 15 hPa. Rrs was measured by the forced oscillation technique (4-32 Hz). End-expiratory lung volume (EELV) and tidal volume (VT) were measured by whole body plethysmography. Rrs was extrapolated to 0 Hz (R₀) and estimated at 16 Hz (R₁₆) by linear regression analysis of Rrs vs. frequency. Specific Rrs, SR₀ and SR₁₆, were then calculated as R₀ (EELV + VT/2) and R₁₆ (EELV + VT/2), respectively. EELV significantly decreased, whereas R₀, R₁₆, SR₀, and SR₁₆ significantly increased, as the CNAP level decreased (P < 0.0001 for all). At the lowest CNAP level, R₀ and R₁₆ reached 198 ± 13 and 175 ± 9% of their respective basal values. The CNAP-induced increase in R₀ was significantly higher than that in R₁₆ (P < 0.004). Our results demonstrate that the CNAP-induced increase in Rrs does not result from a direct lung volume effect only and strongly suggest the involvement of other factors affecting both intrathoracic and extrathoracic airway caliber.