Yuan, Huichin, Edward P. Ingenito, and Béla Suki.Dynamic properties of lung parenchyma: mechanical contributions of fiber network and interstitial cells. J. Appl. Physiol. 83(5): 1420–1431, 1997.—We investigated the contributions of the connective tissue fiber network and interstitial cells to parenchymal mechanics in a surfactant-free system. In eight strips of uniform dimension from guinea pig lung, we assessed the storage (G′) and loss (G”) moduli by using pseudorandom length oscillations containing a specially designed set of seven frequencies from 0.07 to 2.4 Hz at baseline, during methacholine (MCh) challenge, and after death of the interstitial cells. Measurements were made at mean forces of 0.5 and 1 g and strain amplitudes of 5, 10, and 15% and were repeated 12 h later in the same, but nonviable samples. The results were interpreted using a linear viscoelastic model incorporating both tissue damping (G) and stiffness (H). The G′ and G” increased linearly with the logarithm of frequency, and both G and H showed negative strain amplitude and positive mean force dependence. After MCh challenge, the G′ and G” spectra were elevated uniformly, and G and H increased by <15%. Tissue stiffness, strain amplitude, and mean force dependence were virtually identical in the viable and nonviable samples. The G and hence energy dissipation were ∼10% smaller in the nonviable samples due to absence of actin-myosin cross-bridge cycling. We conclude that the connective tissue network may also dominate parenchymal mechanics in the intact lung, which can be influenced by the tone or contraction of interstitial cells.