The establishment of the cardiovascular system represents an early, critical event essential for normal embryonic development. An important component of vascular ontogeny is the differentiation and development of the endothelial and endocardial cell populations. This involves, at least in part, the expression and function of specific cell surface receptors required to mediate cell-cell and cell-matrix adhesion. Platelet endothelial cell adhesion molecule-1 (PECAM-1, CD31) may well serve such a function. It is a member of the immunoglobulin superfamily expressed by the entire vascular endothelium in the adult. It is capable of mediating adhesion by a heterophilic mechanism requiring glycosaminoglycans, as well as by a homophilic, glycosaminoglycan independent, mechanism. It has been shown to regulate the expression of other adhesion molecules on naive T cells. This report documents by RT-PCR and immunohistochemical analysis the expression of PECAM-1 during early post implantation mouse embryo development. PECAM-1 was expressed by early endothelial precursors first within the yolk sac and subsequently within the embryo itself. Interestingly, embryonic PECAM-1 was expressed as multiple isoforms in which one or more clusters of polypeptides were missing from the cytoplasmic domain. The sequence and location of the deleted polypeptides corresponded to exons found in the human PECAM-1 gene. The alternatively spliced isoforms were capable of mediating cell-cell adhesion when transfected into L-cells. The isoforms differed, however, in their sensitivity to a panel of anti-PECAM-1 monoclonal antibodies. These data suggest that changes in the cytoplasmic domain of PECAM-1 may affect its function during cardiovascular development, and are consistent with our earlier report that systematic truncation of the cytoplasmic domain of human PECAM-1 resulted in changes in its ligand specificity, divalent cation and glycosaminoglycan dependence, as well as its susceptibility to adhesion blocking monoclonal antibodies. This is the first report of naturally occurring alternatively spliced forms of PECAM-1 having possible functional implications.