Androgens exert their key function in development and maintenance of the male phenotype via the androgen receptor (AR). Ligand-activated ARs also play a role in prostate cancer. Despite initial success of treatment by testosterone depletion or blocking of androgen binding to the AR using antiandrogens, eventually all tumors escape to a therapy resistant stage. Development of novel therapies by other antagonistic ligands or compounds that target events subsequent to ligand binding is very important. Here, we validate a fluorescence resonance energy transfer (FRET) based imaging assay for ligand-induced AR activity, based on the conformational change in the AR caused by interaction between the FQNLF motif in the N-terminal domain and the cofactor binding groove in the ligand-binding domain (N/C-interaction). We test the assay using known agonistic and antagonistic ligands on wild type AR and specific AR mutants. Our data show a strong correlation between the ligand-induced AR N/C-interaction and transcriptional activity in wild type AR, but also in AR mutants with broadened ligand responsiveness. Moreover, we explore additional readouts of this assay that contribute to the understanding of the working mechanism of the ligands. Together, we present a sensitive assay that can be used to quantitatively assess the activity of agonistic and antagonistic AR ligands. © 2013 International Society for Advancement of Cytometry.