The effect of estrogen, progesterone, testosterone, and hydrocortisone on prostacyclin, thromboxane, prostaglandin E, and prostaglandin F from cultured endothelial cells was investigated. No effect was found on prostacyclin, prostaglandin E, and prostaglandin F. Estrogen increased the release of thromboxane. The significance of this finding with regard to venous thrombolic events associated with estrogen use is discussed.