Although an antiestrogen treatment for estrogen-dependent diseases, such as breast cancers, has been reported, the effect of this endocrine therapy on prolactinomas and its possible mechanism are unclear. This study investigates the antitumor effect of fulvestrant, which is a new estrogen receptor antagonist, on rat prolactinoma MMQ cells and the possible roles of the Wnt/beta-catenin signaling pathway that is involved in this antitumor effect. To investigate the antitumor effect of fulvestrant, the effects of exposure to gradient doses of fulvestrant (0, 0.04, 1, 25, and 625 nM) on the proliferation of cells and the secretion of prolactin (PRL) were studied. Then, the expression levels of the Wnt/beta-catenin signaling pathway-related proteins beta-catenin and Wnt inhibitory factor-1 (WIF-1) were measured to investigate their possible roles in the antitumor effect of fulvestrant. The cells were also treated with decitabine (10 mu M) to investigate the epigenetic mechanism of WIF-1 expression. The proliferation of MMQ cells and the secretion of PRL were suppressed by fulvestrant in a dose-dependent manner (up to 57.0 +/- 3.9 % and 51.2 +/- 4.9 %, respectively). beta-Catenin expression was downregulated and was positively correlated with ER-alpha expression (P < 0.01). As a tumor suppressor, WIF-1 expression was upregulated and was negatively correlated with ER-alpha expression (P < 0.01). Furthermore, WIF-1 expression was upregulated via the hypomethylation of the promoter by decitabine, and cellular proliferation was correspondingly suppressed (37.8 +/- 4.3 %). Antitumor effect of fulvestrant was partially disrupted by SB 216763 via activation of the Wnt/beta-catenin pathway. In conclusion, through the Wnt/beta-catenin signaling pathway, fulvestrant can suppress the proliferation of MMQ cells and the secretion of PRL.