The ligand-induced conformation of a nuclear receptor ligand-binding domain (LBD) is a principal factor leading to transcriptional activity and determining the pharmacological response. Using the estrogen receptor (ER) LBD-labeled site specifically with a fluorophore, we demonstrate that effects of ligand binding on the conformation and dynamics of this domain can be studied directly, in a quantitative and convenient fashion, by various fluorescence methods. Estrogen ligands of different pharmacological character-agonists, selective ER modulators (SERMs), and pure antagonists-each produce distinctive spectroscopic signatures, characteristic of the conformational or dynamic features of their ER-LBD complexes. We can directly follow the equilibrium of helix 12 positions through the degree of local fluorophore rotational freedom and receptor helicity near the C terminus of helix 11. We observe differences even between ligands within a specific pharmacological class, such as the SERMs raloxifene and trans-4-hydroxytamoxifen, highlighting the ability of these fluorescent receptor sensors to detect unique ER conformations induced even by closely related ligands, yet ones that produce distinctive biological activities in estrogen target cells. Fluorophore-labeled LBDs can serve as versatile molecular sensors predictive of ligand pharmacological character and should be broadly applicable to other members of the nuclear receptor superfamily.