Method for determining the ability of a molecule to modulate the activation of a G protein coupled receptor (GPCR), said molecule comprising the following steps: a) introducing, into a first container: - a membrane preparation carrying one or more GPCR and one or more G protein(s), - a GDP source or a GTP source, - a first ligand of the alpha subunit of a G protein (Galpha protein) labelled by a first member of a pair of RET partners, and of a second ligand of the Galpha protein labelled by a second member of the pair of RET partners, said ligands being able to bind specifically, either individually or in association, to the Galpha protein which is not bound to GTP or GDP or to non-hydrolysable or slowly hydrolysable GTP (empty G alpha protein); b) measurement of the RET signal emitted in the first container; c) introduction (i) into a second container, of the same reagents as in step a) and of the molecule to be tested or (ii) into the first container, of the molecule to be tested; d) measurement of the RET signal emitted in the second container or in the first container obtained in step c); e) comparing the signals obtained in steps b) and d); - an increase in the signal when using a GDP source and ligands able to bind specifically to the empty Galpha protein indicating that the molecule is a GPCR agonist; - a decrease in the signal when using a GDP source and ligands able to bind specifically to the empty Galpha protein indicating that the molecule is an inverse GPCR agonist; - an increase in the signal when using a GTP source and ligands able to bind specifically to the empty Galpha protein indicating that the molecule is a GPCR agonist; - a decrease in the signal when using a GTP source and ligands able to bind specifically to the empty Galpha protein indicating that the molecule is an inverse GPCR agonist; characterised in that the first ligand is selected from an antibody, an antibody fragment, a peptide or an aptamer and the second ligand is selected from an antibody, an antibody fragment or an aptamer. a) introducing, into a first container: - a membrane preparation carrying one or more GPCR and one or more G protein(s), - a GDP source or a GTP source, - a first ligand of the alpha subunit of a G protein (Galpha protein) labelled by a first member of a pair of RET partners, and of a second ligand of the Galpha protein labelled by a second member of the pair of RET partners, said ligands being able to bind specifically, either individually or in association, to the Galpha protein which is not bound to GTP or GDP or to non-hydrolysable or slowly hydrolysable GTP (empty G alpha protein); - a membrane preparation carrying one or more GPCR and one or more G protein(s), - a GDP source or a GTP source, - a first ligand of the alpha subunit of a G protein (Galpha protein) labelled by a first member of a pair of RET partners, and of a second ligand of the Galpha protein labelled by a second member of the pair of RET partners, said ligands being able to bind specifically, either individually or in association, to the Galpha protein which is not bound to GTP or GDP or to non-hydrolysable or slowly hydrolysable GTP (empty G alpha protein); b) measurement of the RET signal emitted in the first container; c) introduction (i) into a second container, of the same reagents as in step a) and of the molecule to be tested or (ii) into the first container, of the molecule to be tested; d) measurement of the RET signal emitted in the second container or in the first container obtained in step c); e) comparing the signals obtained in steps b) and d); - an increase in the signal when using a GDP source and ligands able to bind specifically to the empty Galpha protein indicating that the molecule is a GPCR agonist; - a decrease in the signal when using a GDP source and ligands able to bind specifically to the empty Galpha protein indicating that the molecule is an inverse GPCR agonist; - an increase in the signal when using a GTP source and ligands able to bind specifically to the empty Galpha protein indicating that the molecule is a GPCR agonist; - a decrease in the signal when using a GTP source and ligands able to bind specifically to the empty Galpha protein indicating that the molecule is an inverse GPCR agonist; characterised in that the first ligand is selected from an antibody, an antibody fragment, a peptide or an aptamer and the second ligand is selected from an antibody, an antibody fragment or an aptamer. - an increase in the signal when using a GDP source and ligands able to bind specifically to the empty Galpha protein indicating that the molecule is a GPCR agonist; - a decrease in the signal when using a GDP source and ligands able to bind specifically to the empty Galpha protein indicating that the molecule is an inverse GPCR agonist; - an increase in the signal when using a GTP source and ligands able to bind specifically to the empty Galpha protein indicating that the molecule is a GPCR agonist; - a decrease in the signal when using a GTP source and ligands able to bind specifically to the empty Galpha protein indicating that the molecule is an inverse GPCR agonist;