Bisphenol A (BPA) has been shown to induce the activation of nuclear estrogen receptor α/β (ERα/β) in both in vitro and in vivo settings. We originally obtained a 4-methyl-2,4-bis(4-hydroxyphenyl)pent-1-ene (MBP), a possible active metabolite of BPA, strongly activating the ERs-mediated transcription in MCF-7 cells with an EC50 of 2.8 nM (i.e., BPA's EC50 = 519 nM). Environmental estrogens can also target G protein-coupled estrogen receptor 1 (GPER1), a membrane-type ER. However, the effects of BPA/MBP on GPER1, have not yet been fully resolved. In this study, we used MCF-7, a ERα/ERβ/GPER1-positive human breast cancer cell line, as a model to investigate the effects of the exposure to BPA or MBP. Our results revealed that at concentrations below 1 nM MBP, but not BPA, downregulates the expression of GPER1 mRNA via upregulated ERβ, and the MCF-7 cells pre-treated with MBP display resistance to GPER1 agonist G-1-mediated anti-proliferative effects. Because GPER1 can act as a tumor suppressor in several types of cancer including breast cancer, the importance of MBP-mediated decrease in GPER1 expression in breast cancer cells is discussed.