We previously studied low-frequency respiratory impedance (Zrs) data at an elevated lung volume to separate airway and tissue mechanical properties in normal infants (Am. J. Respir. Crit. Care Med. 1996; 154:161–166). The aim of the present study was to determine the volume dependence of the airway and tissue mechanics by extending Zrs measurements to lower lung volumes. Zrs spectra between 0.5 and 21 Hz were measured in supine sleeping infants (n = 8; 7 to 26 mo of age) at mean transrespiratory pressures (Ptr_mean) of 20, 10, and 0 cm H₂O, during periods of apnea induced by inflating the infants' lungs to a pressure of 20 cm H₂O through a face mask. At each inflation pressure, a model containing airway resistance (Raw) and inertance (Iaw) and tissue damping (G) and elastance (H) was fitted to Zrs data. At FRC, the values of Raw, Iaw, G, and H were 20.6 ± 4.9 (SD) cm H₂O · s/L, 0.037 ± 0.014 cm H₂O · s²/L, 39.6 ± 10.3 cm H₂O/L, and 147 ± 35 cm H₂O/L, respectively. Increase of Ptr_mean caused a monotonous decrease in Raw (42 ± 7% of the value at FRC), while Iaw remained constant. The tissue parameters were minimal at a Ptr_mean of 10 cm H₂O (68 ± 10% and 78 ± 6% in G and H, respectively) and significantly higher at both 0 and 20 cm H₂O. Although Zrs measurements can be made in most infants at lung volumes as low as FRC, an inflation pressure of 20 cm H₂O provides a higher success rate and is therefore a more suitable condition for general use.