The purpose of this study was to identify an endometrial cell line that maintained the E2 up-regulation of estrogen receptor (ER) mRNA by enhanced message stability and to assess its dependence on ER protein. Estradiol (E2) effects on gene expression were measured in three cell lines: one immortalized from sheep endometrial stroma (ST) and two from human endometrial adenocarcinomas (Ishikawa and ECC-1). E2 up-regulated ER mRNA levels in ST and Ishikawa cells, but down-regulated ER mRNA levels in ECC-1 cells. E2 up-regulated progesterone receptor (PR), glyceraldehyde 3-phosphate dehydrogenase (GAPDH), and transforming growth factor-α (TGF-α) in both Ishikawa and ECC-1 cells. The selective estrogen receptor modulator ICI 182,780 antagonized the E2-induced up-regulation of ER and/or PR mRNA levels in all three cells, while another, GW 5638, antagonized the up-regulation of PR mRNA in Ishikawa and ECC-1 cells. In mechanistic studies, E2 had no effect on ER mRNA stability in ST cells and it destabilized ER mRNA in ECC-1 cells. Thus, Ishikawa cells appear to be the most physiologically relevant cell line in which to study the up-regulation of ER mRNA levels by enhanced mRNA stability. Its antagonism by ICI 182,780 reveals that ER protein is involved in this E2 response.