Viscoelasticity of air-filled isolated lungs of 1-120 day old rabbits was studied at a mean transpulmonary pressure (P1) of 0.65 kPa by measuring: (1) their stress relaxation (SR) following step volume changes (delta V) corresponding to 5% of the vital capacity; (2) their resistance (R1) and elastance (E1) during sinusoidal cycling at 11 frequencies (f) ranging from 0.01 to 0.65 Hz. SR data were analyzed by least-square regression using Hildebrandt's logarithmic model (Hildebrandt, J. (1970) J. Appl. Physiol. 28: 365-372): delta P1/delta V = A - B.log(t) where A is an index of elastance, and B/A is a measure of viscoelasticity. Coefficients A and B were also obtained from R1 and E1 according to: R1 = B/9.2 f and E1 = A + 0.25 B + B.log(2 pi f). Elastance corrected for lung weight increased by 40% between day 1 and days 3-7, and decreased thereafter to reach 30% of its initial value in 120 day old rabbits. B/A ratios also demonstrated an initial rise, followed by a progressive decrease. Values of B/A computed from R1 and E1 were similar to those derived from SR data in 1 day old rabbits, but were 20-30% larger in older animals, which indicates the presence of an additional rate-independent dissipation during flow. Total internal dissipation during cycling varied little with frequency; it was largest in 3-7 day old animals where it represented 20% of the stored elastic energy.