The cellular action of steroid hormones is mediated by specific receptors. Recently, two different estrogen receptors (ER), alpha and beta, have been cloned with a specific tissue distribution. Active estrogen as well as active progestin are compounds of oral hormonal contraceptives and hormone replacement therapy. To examine the regulation of ER-alpha and -beta activities after treatment with synthetic progestins and synthetic and natural estrogens, COS 7 cells were transfected with the vector expressing ER-alpha and -beta in combination with a luciferase reporter vector. ER-alpha activity was upregulated in the presence of synthetic progestins in a dose-dependent manner. Norethisterone, norethynodrel and desogestrel proved to be the most potent stimulatory agents of ER-alpha expression. On the other hand, not all progestins exhibited a stimulatory action on ER-beta activity. Only norgestrel, levonorgestrel, norethynodrel and norethisterone induced ER-beta-activating functions in a dose-dependent manner. Luciferase activity due to estrogen stimulation served as a positive control. Our results indicate that progestins have different effects on the activities of ER-alpha and -beta.