This study was designed to investigate the local changes in the levels of placental estradiol and progesterone and their ratio during term labor and to determine whether the PGE2-generating ability of the fetal membrane in response to IL-1 beta at term labor is greater than that at term not-in-labor. Forty pregnant women were divided into two groups: term labor and term not-in-labor. Placental estradiol and progesterone were measured by radioimmunoassay. The levels of fetal membrane PGE2 were measured by enzymoimmunoassay. The results showed that the placental concentration of estradiol and progesterone remained unchanged at the onset of labor, but the ratio of estradiol to progesterone increased significantly (P < 0.05). IL-1 beta stimulated fetal membrane to produce more PGE2 at term labor, and at term not-in-labor, too. But the increment of PGE2 generated by fetal membrane at term labor was greater than that at term not-in-labor. It is concluded that the change in placental estradiol to progesterone ratio may play an important role in the initiation of labor by altering the PGE2-generating ability of fetal membrane in response to cytokines.