The synthesis of prostaglandins (PGs) was determined in endothelial cells obtained from various vessels from baboon, human and rat both by radioimmunoassay and prelabel of the cells with [3H] arachidonate. Cells were stimulated with bradykinin, ionophore A23187 or 10 microM arachidonate. Although prostacyclin (PGI2) has proven to be the major prostaglandin product of human umbilical vein and calf pulmonary artery endothelial cells, our results show that PGI2 is frequently not the major prostaglandin product of endothelial cells from other vessels. For example baboon endothelial cells lining the large vessels, aorta and cephalic vein produce mainly PGF2a with only small amounts of PGE2 and PGI2. Human endothelial cells from saphenous vein also produce mainly PGF2a. Baboon, human and rat adipose capillary endothelial cells make predominantly PGE2 and PGI2 with rat making significant amounts of PGF2a in addition. Endothelial cells from the rat aorta produced predominantly prostacyclin.