Respiratory input impedance (Zin) is a potentially informative test of pulmonary function in infants who are unable to perform standard tests commonly performed in children and adults Analysis of Zin in dogs using the six-element model of DuBois et al. (J Appl Physiol 8:587, 1956) provides estimates of airways resistance separate from tissue resistance, as well as an estimate of thoracic gas volume. However, reliable estimates of these parameters can only be obtained when Zin displays a distinct antiresonance that is associated with the tissue inertance and alveolar gas compression compliance. To determine whether infants have such an antiresonance. Zin was measured in nine healthy infants (4 < f < 160 Hz). An antiresonance was found at 112.8-10.4 Hz, and the six-element model fit these data well, but the resulting parameters were physiologically unrealistic. We hypothesized that the antiresonance in the measured Zin is the result of shunt compliance proximal to alveolar gas compression compliance. Gas compression in the face mask and nonrigid upper airway walls could provide such a shunt compliance. We investigated another model with four parameters, a single shunt compliance (Cim) representing gas compression in the face mask in parallel with the infant's total respiratory resistance (Rrs) inertance (Irs), and compliance (Crs). This model fits the data well, and the estimated R, (19.3, 4.2 cmH O/L/s) was physiologically reasonable. However, Crs (Crs 1.03-0.58 mL cmH2O) was one order of magnitude smaller than reported Crs. The value for Cim was slightly larger than that based on the estimated volume of gas in the face mask, suggesting an additional influence of upper airway wall shunting. Computer simulations using a model that includes the face mask and upper airway walls confirmed that Cim and the upper airway wall properties significantly influence Zin data over this frequency range. Nevertheless, these simulations suggest that the Rrs estimated from the four-element model is related to airway resistance.