Administration of anti-inflammatory non-steroidal drugs to pregnant animals causes constriction of the fetal ductus arteriosus (DA). To clarify the mechanism of the indomethacin-induced constriction of the rat fetal DA, extracellular matrix (ECM) components were observed by immunohistochemical techniques with antibodies 2B1, 6B6, 3B3, 9A2 and others to various kinds of proteoglycans (PGs). In the 10mg/kg group, the large PG revealed with antibody 2B1 was increased in the inner and outer media at constriction, and chondroitin 6-sulfate PG was strongly immunostained with antibody 3B3 as granular substances in the outer media, although no difference in stainability of chondroitin 4-sulfate PG revealed with the antibody 9A2 was observed between experimental and control groups. This suggests that the cells composing the fetal DA showed a tendency of proliferation and resultant production and aggregation of large PG with chondroitin 6-sulfate chains during constriction induced by indomethacin-treatment.