When the Hering-Breuer reflex is used to relax the respiratory muscles, the passive compliance (Crs), resistance (Rrs) and time constant (Trs) of the respiratory system can be measured from the subsequent expiration. This method was used to assess 22 newborns with respiratory illness. Passive expirations were also recorded in 6 paralyzed, ventilated children. Using a simple slide valve, airway occlusion pressure was measured from a face mask or endotracheal tube, and expiratory flow was measured from a pneumotachygraph. In all subjects, there was a linear function of expiratory flow vs. its integral, volume; by extrapolating the linear function to 0 flow and 0 volume (i.e., initial volume at time 0) Crs, Rs and Trs could be calculated; Crs was significantly reduced in mechanically ventilated vs. spontaneously breathing infants, and Rrs was significantly higher in intubated vs. extubated infants. During passive expiration in newborns, inspiration occurred at a volume above passive functional residual capacity. The passive expiratory flow-volume technique is simple, noninvasive and appears to provide accurate measurements of respiratory mechanics in the newborn and in paralyzed children.