Angiotensin II (ANG II), generated by activation of local renin-angiotensin systems, is believed to play an important role in tissue repair and remodeling, in part via transforming growth factor-β (TGF-β). Angiotensin-converting enzyme (ACE) inhibitors have been shown to abrogate experimental lung injury via a number of potential mechanisms; however, the potentially fibroproliferative role for ANG II in the lung has not been characterized. We hypothesized that, after lung injury, ANG II would stimulate fibroblast procollagen synthesis and promote lung collagen deposition in rats. In vitro, ANG II was a potent inducer of procollagen production in human lung fibroblasts via activation of the type 1 receptor and, at least in part, via the autocrine action of TGF-β. After bleomycin-induced lung injury, an increase in lung ANG II concentration was observed by day 3 that preceded increases in lung collagen and was maintained until death at day 21. Administration of an ACE inhibitor (ramipril) reduced ACE activity, ANG II concentration, TGF-β expression, and collagen deposition. Losartan (an ANG II type 1 receptor antagonist) also attenuated the increase in TGF-β expression and lung collagen deposition. These observations suggest that ANG II, possibly generated locally within the lung, may play an important role in the fibrotic response to acute lung injury, at least in part via the action of TGF-β. ACE inhibitors and receptor antagonists, already widely used clinically, should be assessed as potential new therapies for fibrotic lung disease.