Ex v/vo addition of estradiol 1713 to first trimester or term human placental minces caused a significant increase in the quantity of progesterone produced. Addition of an aromatase inhibitor, CGS 16949 A, or the estrogen receptor antagonist, ICI 182780, significantly inhibited progesterone production confirming the role of estradiol 1713 in the regulation of progesterone synthesis in human pla...