Established clonal strains of rat pituitary cells, GH-cells, responded prior to 1974 to 10(-11) to 10(-8)M 17beta-estradiol by increasing prolactin synthesis 2-fold and decreasing the production of growth hormone to between 20 and 70% of control values. In experiments in 1975-76, these effects of estradiol were no longer statistically significant. Unexpectedly, the estrogen antagonists enclomiphene, CI-628, tamoxifen, and Lilly 88751 caused increases in both prolactin and growth hormone synthesis at concentrations of about 10(-7) to 10(-6)M after one week of treatment. These increases were prevented by 10(-8)M 17beta-estradiol. Prolactin synthesis remained elevated for at least 11 days after removal of enclomiphene from the culture medium but, in the presence of estradiol, synthesis approached control levels by 11 days. In GH-cells, compounds which are estrogen antagonists in other systems mimic the previously observed effect of estradiol on prolactin synthesis, but have an effect opposite to that of estradiol on growth hormone, namely stimulation of its synthesis.