Glomerular changes in patients with cyanotic congenital heart and chronic lung diseases and in persons living at a high altitude might be related to hypoxemia. This study was carried out to examine the effects of hypoxia on the extracellular matrix production by cultured rat mesangial cells (CRMC). Subconfluent CRMC monolayers were grown with 10% oxygen (hypoxia) and 20% oxygen (control) for 1, 3, and 5 days. The production of type IV collagen (CIV), fibronectin (FN), and laminin (LN) by CRMC was evaluated by flow cytometry and immunofluorescent microscopy. Total RNA was extracted and Northern blotting and hybridization were performed with cDNAs for CIV, FN, and LN. The surface expression of CIV on FC was higher in hypoxia than under control conditions at day 5 (158% of control). The surface expression of FN was also higher in hypoxia at day 3 (303%) and at day 5 (332%). The surface expression of LN was lower at day 1 (71%). Immunofluorescent microscopy showed similar changes with flow cytometry. The mRNA level for CIV and FN was maximal at day 5 with 206 and 305% of control, respectively. Hypoxia had little effect on LN mRNA expression. These results show that hypoxia stimulates the synthesis of extracellular matrix of cultured rat mesangial cells. Hypoxia may contribute to the development of glomerular changes in cyanotic congenital heart diseases, chronic lung diseases, and in persons living at a high altitude.