The expiratory time constant of the respiratory system (Trs) and the total respiratory system compliance (Crs) were measured from the analysis of a flow-volume loop during relaxed expiration, and the total respiratory system resistance (Rrs) was derived from the relationship: Rrs = Trs/Crs. In ten intubated and mechanically ventilated very low birth weight infants, the values obtained for compliance (0.21 to 0.71 ml/cm H2O) were similar to those found using an occlusion technique during spontaneous breathing. The values of Trs were 0.03 to 0.14 sec. These Trs values were altered by adding known resistive loads. The corresponding derived values for Rrs were close to the values of the added resistance loads, suggesting that resistance values found in this group of ventilated babies (120 to 380 cm H2O/L X sec) were accurate.