Fibrosis is the final common pathway for various tissue lesions that lead to chronic progressive organ failure, and consequently effective antifibrotic drugs are strongly desired. However, there are few animal models in which it is possible to evaluate fibrosis sensitively in a short period of time. We therefore generated two transgenic rats harboring a firefly luciferase reporter gene under the control of the 5'-flanking region of rat α(1)(I) collagen (Col1a1-Luc Tg rats) and α(2)(I) collagen (Col1a2-Luc Tg rats). The luciferase activities of these transgenic rats were highly correlated with the hydroxyproline content in various organs. In unilateral ureteral obstruction (UUO), a well-characterized model of renal fibrosis, the luciferase activity in obstructed kidneys showed a significant increase after even 3 days of UUO, while the hydroxyproline content showed little increase. In addition, the renal hydroxyproline content had a higher correlation with the luciferase activity than α(1)(I) collagen mRNA level for over 2 wk after UUO. Although both an ANG II type 1 receptor blocker (ARB), olmesartan, and a transforming growth factor-β (TGF-β) type I receptor kinase (ALK5) inhibitor, SB-431542, inhibited renal luciferase activities in UUO, only SB-431542 inhibited luciferase activity induced by TGF-β1 in isolated glomeruli. Double immunostaining for luciferase and α-smooth muscle actin (α-SMA) revealed that some α-SMA-positive tubular epithelial cells and tubular interstitial cells produced type I collagen, which would lead to renal fibrosis. Thus collagen reporter transgenic rats would be very useful for the evaluation of antifibrotic effects and analysis of their mechanisms.