Estrogen (E) regulates the synthesis and secretion of several pituitary hormones during the reproductive cycle in a cell- and promoter-specific manner. One mechanism underlying cell specificity is the differential expression of estrogen receptor (ER) isoforms. We used in viva and in vitro rodent pituitary cell models to examine the expression and regulation of ERα, ERβ, and the pituitary-specific ERα isoform, truncated estrogen receptor product-1 (TERP-1). In cycling female rat pituitaries, ERβ messenger RNA (mRNA) levels fell 40% on the morning of proestrus and were suppressed by E or dihydrotestosterone in ovariectomized females. In lactotrope and gonadotrope cell lines (GH3, RC4B, LβT2), progesterone (P) or P plus E also suppressed ERβ. TERP-1 mRNA increased 3-fold at proestrus and in response to E treatment in viva and in cell lines. ERα mRNA levels were not regulated significantly by any treatment in vivo or in cell lines. However, E suppressed ERa protein levels in vivo and in cell lines, and reduction of ERa protein levels by E or the antiestrogen ICI182,780 reduced E-stimulated transcriptional activation of the PRL promoter in GH3 cells. TERP-1 and ERβ protein levels were low to undetectable in cell lines, but E stimulated TERP-1. Because E treatment decreases ERβ mRNA and ERα protein and increases levels of TERP-1 (which can suppress ERα/β activity), the dynamic steroid-induced changes in ER expression in the rat pituitary during the midcycle gondaotropin/PRL surge may provide a means for ovarian steroids to limit positive feedback.