The contractile capability of lung parenchymal strips isolated from normal rats was compared with that of strips isolated from rats with pulmonary fibrosis induced by intratracheal instillation of bleomycin sulfate. Subsequently, the population of contractile cells, both muscle and nonmuscle, was analyzed in each strip by histochemical, immunocytochemical, and ultrastructural means. The force (g force/g tissue wet weight) generated by fibrotic strips was approximately double that of the control strips in response to acetylcholine, epinephrine, and potassium depolarizing solution. The content of smooth muscle in fibrotic and control strips was not significantly different. Immunofluorescent examination indicated increased contractile proteins (actin and myosin) in the thickened alveolar walls of the fibrotic strips in areas devoid of histologically demonstrable smooth muscle. Ultrastructural examination of the fibrotic interstitium revealed an increased population of filament-laden cells that appeared related to the contractile interstitial cell, or myofibroblast. The results indicate that parenchymal strips isolated from fibrotic rat lung can generate increased force in vitro. These responses may be related to increased nonmuscle contractile cells in the fibrotic interstitium.