Arsenic (As) is considered a top environmental chemical of human health because it has been linked to adverse health effects including cancer, diabetes, cardiovascular disease, and reproductive and developmental problems. In several cell culture and animal models, As acts as an endocrine disruptor, which may underlie many of its health effects. Previous work showed that steroid receptor (SR)-driven gene expression is disrupted in cells treated with inorganic As (arsenite, iAs+3). In those studies, low iAs+3 concentrations (0.1–0.7 μM) stimulated hormone-inducible transcription, whereas somewhat higher but still non-cytotoxic levels (1–3 μM) inhibited transcription. This investigation focuses on the mechanisms underlying these inhibitory effects and evaluates the role of methylated trivalent As metabolites on SR function. Recent evidence suggests that, compared with iAs, methylated forms may have distinct biochemical effects. Here, fluorescence polarization (FP) experiments utilizing purified, hormone-bound human glucocorticoid (GR) and progesterone receptor (PR) have demonstrated that neither inorganic (iAs+3) nor dimethylated (DMA+3) species of trivalent As affect receptor interactions with glucocorticoid DNA response elements (GREs). However, monomethylated forms (monomethylarsenite, MMA+3 and monomethylarsonic diglutathione, MADG) strongly inhibit GR-GRE and PR-GRE binding. Additionally, speciation studies of iAs+3-treated H4IIE rat hepatoma cells show that, under treatment conditions that cause inhibition of hormone-inducible gene transcription, the intracellular concentration of MADG is sufficient to inhibit GR-GRE and PR-GRE interactions in vivo. These results indicate that arsenic's inhibitory endocrine disruption effects are probably caused in part by methylated metabolites' disruption of SR ability to bind DNA response elements that are crucial to hormone-driven gene transcription. Copyright © 2013 John Wiley & Sons, Ltd.