Leptin is a potential regulator of conceptus development. We have previously suggested that in primate pregnancy, leptin biosynthesis is regulated by estrogen in a tissue-specific manner. Therefore, the objective of the current study was to determine the mechanism of estrogen action on LEP promoter activation in divergent cell types. The effects of estrogen were investigated in estrogen receptor (ER)-positive MCF-7 breast cancer cells and in ER-negative JEG-3 choriocarcinoma cells. Cells were transfected with a leptin-luciferase or an estrogen responsive element (ERE)-luciferase reporter construct, in conjunction with ERalpha, ERbeta, or empty vector expression plasmids. Cells were treated with estradiol and/or the specific estrogen antagonists, ICI-182,780 or 4-hydroxytamoxifen. In MCF-7 cells, estradiol stimulated (P<0.05) ERE-luciferase activity and was inhibited by ICI-182,780, but did not stimulate leptin-luciferase activity. However, leptin-luciferase was stimulated by estradiol (P<0.05) and inhibited by antiestrogens in JEG-3 cells that were co-transfected with ERalpha. Both antiestrogens stimulated leptin-luciferase activity (P<0.05) in JEG-3 cells co-transfected with ERbeta. Results suggested that LEP promoter activation may depend upon co-activators present in leptin-producing cells and may be inhibited by repressors present in non-leptin producing cells. Divergent effects of estrogen may be owed to differences in the type of ER (alpha or beta) expressed in target tissues.