We have used the polymerase chain reaction to identify mouse proteins similar in primary structure to the endothelial cell mitogen Vascular Endothelial Growth Factor (VEGF). One amplified product encoded mouse Placenta Growth Factor (P1GF). The pattern of P1GF gene expression in mouse embryos was studied by in situ hybridization. Transcripts encoding mouse P1GF were abundant in trophoblastic giant cells associated with the parietal yolk sac at early stages of embryogenesis. VEGF transcripts were also detected in trophoblastic giant cells raising the possibility that these cells may secrete heterodimers consisting of one P1GF subunit and one VEGF subunit. The secretion of P1GF and VEGF by trophoblastic giant cells is likely to be the signal which initiates and co-ordinates vascularization in the deciduum and placenta during early embryogenesis.