Labelling with endothelium specific monoclonal antibodies, von Willebrand Factor (vWF), rat endothelial cell antigen-1 (RECA-1), platelet-endothelial cell adhesion molecule-1 (PECAM-1), intercellular adhesion molecule-1 (ICAM-1). OX-43 and zonula occludentes-1 (ZO-1), was investigated in cryostat sections of vessels From rats of different ages using a confocal microscope. The results showed that labelling of the vWF was positive in endothelial cells from adult, fetal and different ages of embryonic rat. Labelling with RECA-1 was weakly positive in adult rat aorta and lung endothelial cells but not in embryonic yolk sac endothelial cells. Labelling using PECAM-1. ICAM-I and OX-43 as negative in both adult and embryonic endothelial cells. ZO-1 showed positive but very weak reactivity in embryonic yolk sac endothelial cells. The expression of vWF on vessels from adult and 19.5-day Fetal tissues was strongly positive, However, the expression of vWF in embryonic endothelial cells was dependent on the gestational age, While the 11.5-day yolk sac vessels stained weakly, staining gradually increased in 13.5-. 15.5- and 17.5-day-old yolk sac vessels. The results Suggest that vWF is a reliable endothelial cell marker in rat vascular endothelial cells, including both fetal and embryonic stages.