Abstract: Melatonin, via its MT1 receptor, but not the MT2 receptor, can modulate the transcriptional activity of various nuclear receptors – estrogen receptor alpha (ERα) and retinoic acid receptor alpha (RARα), but not ERβ– in MCF-7, T47D, and ZR-75-1 human breast cancer cell lines. The anti-proliferative and nuclear receptor modulatory actions of melatonin are mediated via the MT1 G protein-coupled receptor expressed in human breast cancer cells. However, the specific G proteins and associated pathways involved in the nuclear receptor transcriptional regulation by melatonin are not yet clear. Upon activation, the MT1 receptor specifically couples to the Gαi2, Gαi3, Gαq, and Gαll proteins, and via activation of Gαi2 proteins, melatonin suppresses forskolin-induced 3′,5′-cyclic adenosine monophosphate production, while melatonin activation of Gαq, is able to inhibit phospholipid hydrolysis and ATP’s induction of inositol triphosphate production in MCF-7 breast cancer cells. Employing dominant-negative and dominant-positive) forms of these G proteins, we demonstrate that Gαi2 proteins mediate the suppression of estrogen-induced ERα transcriptional activity by melatonin, while the Gq protein mediates the enhancement of retinoid-induced RARα transcriptional activity by melatonin. However, the growth-inhibitory actions of melatonin are mediated via both Gαi2 and Gαq proteins.