The frequency dependence of total respiratory impedance during spontaneous breathing was measured repeatedly in 16 children (3--5 yr old) over a 3-mo period using forced random noise and spectral analysis. Total respiratory resistance, compliance, and inertance, which were calculated fromthe impedance data using regression analysis with a second-order model, had overall mean values +/- SD of 5.61 +/- 0.49 cmH2O.l-1.s, 403 +/- 1.04 ml.cmH2O-1, and 0.0120 +/- 0.0024 cmH2O.l-1.s2, respectively. Linear regression analysis showed that resistance and compliance correlated with either height or forced vital capacity with r values in the range 0.77--0.89. Analysis of measurement variability suggested that resistance measurements had, on the average, a coefficient of variation of 15%; corresponding values for compliance and inertance measurements were 25%.