The non-steroidal antiestrogen, tamoxifen, is now accepted as a useful therapeutic agent for the treatment of hormone-dependent breast cancer. However, the pharmacology of antiestrogens has not been systemically investigated and their molecular mechanism of action is unknown. To address these problems, we have developed an estrogen-sensitive pituitary cell culture system. The structure-activity relationship of antiestrogenic ligands in the modulation of prolactin synthesis in rats and mice will be studied following either in vivo treatment of animals or addition of compounds to cultured rat and mouse pituitary cells. We will investigate whether the profound species differences in the action of antiestrogens (rats vs mice) are due to altered ligand requirements or to differential drug metabolism. The basis for the opposite effects on prolactin synthesis of estrogens and antiestrogens GH3 clonal rat pituitary tumor cells - compared to normal pituitary cells - will be studied. In selected experiments, the relative abilities of antiestrogens to inhibit prolactin synthesis and induce progesterone receptors will be compared. Since carcinogen-induced rat mammary cancer is known to be prolactin dependent, our studies will provide precise data on an antitumor mechanism of antiestrogens.