The purpose of this study was to compare the work of breathing during pressure support ventilation (PSV) with positive end expiratory pressure (PEEP) utilizing the Siemens SV300® (SV300) and Dräger Evita 4® (EV4) ventilators. Our hypothesis was that patients' work of breathing (WOBP) would be unchanged in PSV utilizing flow triggering (FT) in both the SV300® and EV4®. We compared two ventilators using six healthy, intubated, sedated, spontaneously breathing pigs weighing approximately 10 kg each. WOBP (j/L) and ventilator work of breathing (WOBV) (j/L) were measured using a portable monitor which utilizes an esophageal balloon and flow transducer. Each breath was further analyzed for duration of inspiratory effort and negative deflection of pressure needed to trigger PSV. Animals were studied with the SV300® and EV4® on a pressure support of 5 cmH2O and PEEP settings of 0 and 5 cmH2O. Data were analyzed using the Wilcoxon signed rank test with significance set at P ≤ 0.05 WOBP was 90% (PS 5, PEEP 0) and 52% (PS 5, PEEP 5) lower on the SV300® compared to the EV4®. WOBV was 94% (PS 5, PEEP 0) and 39% (PS 5, PEEP 5) higher on the SV300® when compared to the EV4®. The change in airway pressure (delta p) from baseline and most negative deflection of pressure were greater with the EV4® as compared to the SV300®, although delta pressure was not found to be statistically significant on PS of 5 cmH2O and PEEP of 5 cmH2O. The SV300® also had shorter duration of inspiratory effort from initiation of breath to most negative deflection of pressure and to maximum flow than the EV4®. In conclusion, these results suggest there are significant differences in WOBP between the SV300® and EV4® ventilators. Response time of the ventilators may explain the differences in duration of inspiratory effort and the patient's work of breathing and thus may have an impact on weaning time for ventilated patients. Pediatr Pulmonol. 2001; 32:62–70. © 2001 Wiley‐Liss, Inc.