Many recent ventilators used in the ICU have a demand valve for intermittent gas flow. While demand systems conserve gas flow, they do not appear to be as well tolerated by patients as are continuous flow systems. Using a mechanical model to simulate a patient's spontaneous breathing efforts, we tested nine common ventilators in the intermittent mode. Their sensitivity varied from −2.0 to −0.2 cm H2O and the total delay time from 19 to 92 msec. The initial inspiratory resistance varied between 0.03 and 0.10 cm H2O/L min and initial flow was between 2.5 and 12.3 L/min. These values were often not supplied or not the same as those in the manufacturer's specifications.