The ARDS is a clinical entity which from a mechanical point of view is characterized not only by a low respiratory compliance but also by a high respiratory resistance (R). In five ARDS patients we analyzed the changes in total R of the respiratory system (Rmax) and airway plus endotracheal tube R (Rmin) induced by changing the airflow (V) rate at constant tidal volume (VT) and total PEEP (PEEP plus autoPEEP). Patients were ventilated in volume controlled mode (VC) and VC with inverse ratio (VCIRV), both with constant insufflation flow. Signals of V, and airway pressure (Paw) were digitized (sampling frequency 200Hz) and acquired by an IBM 55SX computer in order to perform the following calculations: Rmax, Rmin, DR (Rmax-Rmin), static compliance (C). VT was obtained by digital integration of V. Rmin was corrected for the closing time of the inspiratory valve. Statistical analysis was performed with a two-way ANOVA. Results: V PEEPt Ti/Tt VT L/e cmH20 8 reL VC .76±.06 9.5±.4 34+.5 590±42 VCIRV .30+.02 8.4+.5 73+2 610±47 P= <.001 .15 <.001 .5