This study was designed to investigate the effects of 17 beta-estradiol on prostanoid formation from exogenous arachidonic acid in homogenates of human umbilical cord vessels. The veins produced more prostanoids than the arteries and predominantly 6-keto-PGF1 alpha. 17 beta-estradiol had no effect on the rate of production of prostanoids in either vessels. Thus, at least in our in vitro system, the regulation of the vascular tone by prostanoids seems not to be altered by the addition of 17 beta-estradiol.