In order to assess the role of collagens Iand IV during the angiogenic processassociated with bleomycin-induced pulmonaryfibrosis in rat, in situ hybridization andimmunocytochemical studies were carried out. An increased expression of collagen IV wasobserved before an enhanced expression ofcollagen I after intratracheal instillation ofbleomycin. Deposits of both collagen typeswere detected on the 21st day after treatmentwith bleomycin, surrounding the new bloodvessels formed during the fibrotic process. Atthis time, the presence of new lymphaticvessels was associated uniquely withdeposition of collagen I. These observationslead us to conclude that, at least duringpulmonary fibrosis, lymphangiogenesis takesplace after blood angiogenesis.