Endometrial stromal differentiation (decidualization) is essential for implantation of the developing blastocyst. Because prostaglandins (PGs) are synthesized in the endometrium, and PG-binding sites have been demonstrated in the proliferative endometrial stromal cell (the precursor of the decidual cell), experiments were performed to determine whether PGs are involved in the process of decidualization. Human endometrial stromal cells were cultured for 18 days in Dulbec-co’s Modified Eagle’s Medium-2% fetal bovine serum with 1 μM medroxyprogesterone acetate (MPA) and 10 nM estradiol, with and without PGE2 or PGF2α. Expression of PRL was used as a marker of decidualization. In the presence of estradiol and MPA alone (control), PRL was detected beginning on day 9 and gradually increased through day 18. In contrast, PRL was detected on day 3 in the PGE2-treated cells, and the magnitude of stimulation in these cells on days 9-12 was 1300-1400% of that in control cells. Furthermore, the PRL mRNA content of the PGE2-treated cells on day 12 was 4.6-fold greater than that in the control cells. The effect of PGE2 on PRL production was dose dependent, with a minimal effective dose of 10-10 M. PGE2 in the absence of steroids had a minimal effect on PRL production. In contrast to PGE2, PGF2α treatment had no effect on PRL expression in steroid-treated cells. These results indicate that there are synergistic effects among PGE2, estradiol, and MPA, resulting in acceleration of endometrial stromal cell differentiation and enhanced PRL expression.