Impedance of the total respiratory system was measured in 121 healthy children aged 4-16 years during spontaneous breathing by pseudo-random forced oscillations between 3 and 10 Hz. Total respiratory resistance (Rrs), inertance (Irs) and compliance (Crs) were determined by least-mean-squares fitting. Estimates for inertance were reliable only for the larger children, where the values of Irs (0.0127 +/- 0.0034 SD) were similar to those reported for normal adults. Rrs correlated significantly (P less than 0.001) with height (r = -0.868), age (r = -0.865), and, in a subpopulation of the 6- to 16-year-old children, with forced vital capacity (r = -0.803). The corresponding correlation coefficients for Crs were 0.873, 0.844, and 0.853, respectively. Crs amounted to about a third of the static total compliance values of Sharp et al. (J Appl Physiol 1970; 29: 775-779) over the same interval of heights. In these relationships no significant difference was found between boys and girls.