Fibulins-1 and -2 are two recently characterized extracellular matrix proteins of a novel protein family. They are found in many tissues in the adult stage, but several differences in their expression patterns were noted in cultured cells and adult tissues. Fibulin-1 is particularly well expressed in the endocardial cushion tissue (ECT) during embryogenesis and we therefore compared the expression patterns of fibulin-1 and -2 in the developing mouse heart. Antibodies against mouse fibulin-1 and -2 were used in immunofluorescence, and the expression of mRNAs were studied with synthetic oligonucleotides by in situ hybridization. A striking codistribution of fibulin-1 and -2 was seen at all stages of the development of the endocardial cushion tissue. Fibulin-2 was found to be a more specific marker for the embryonic ECT than fibulin-1, which was also transiently expressed in cardiac muscle. Immunofluorescence showed that the fibulins remained expressed in valves of the heart in the adult stage. The results suggest a role of the two fibulins in the development of the ECT and in the maintenance of the tensile strength of the cardiac valves.