Knowledge of immature tracheae mechanical behavior is fundamental in understanding the effects exerted on the upper airways by tidal liquid ventilation (TLV). Particularly, negative pressure can take place along the airways during expiration, which can cause airway collapse and flow limitation; therefore, representing a critical issue in preterm infant patients, whose airways are less stiff than adult ones. In this study, we investigated the expiratory pressure drop vs flow relationship of isolated preterm lamb tracheal samples to determine their hydraulic resistance, collapse pressure and collapse flow rate; a liquid flow through the samples was obtained by applying negative pressure at the outlet (cephalad) extremity of the tra-cheal sample, while keeping the inlet (caudal) extremity at atmospheric pressure. Histological analyzes were performed on the tracheal samples after each test session, in order to examine the morphological structure of the tracheal wall. Flow resistance tests demonstrated progressive lumen narrowing at increasing pressure drop (∆P=P in -P out ). The flow rate increased with ∆P un-til a plateau was reached, and then decreased, describing the onset of a collapse phenomenon; however, complete occlusion was not reached. The tracheal samples demonstrated a similar behavior to that of a Starling resistor during the collapse phase: when a critical ∆P was reached, collapse was observed starting at the outlet region, which was subjected to the greatest negative pressure, then propagating towards the caudal direction. (Journal of Applied Biomaterials & Biomechanics 2004; 2: 177-82).