The present invention relates to novel biosensors that are based on bioluminescence resonance energy transfer (BRET). These biosensors may be used to monitor rapid interaction and conformational changes within G protein-coupled receptor/G protein complexes and, in this way, reflect the activation status of the receptor. Advantageously, the biosensors may be used as a highly sensitive and quantitative assay for the identification of ligands (agonists, antagonists, inverse agonists, partial agonists, etc.) targeting G protein-coupled receptors (GPCRs) as well as for the analysis of the activation status of these receptors. Moreover, multiplexing different biosensors within receptors/G protein complexes allows for mapping ligand textures. Additionally, the biosensors permit the direct, real-time examination of interactions between receptors and G protein in their natural environment, the living cell.