Several different approaches have been applied in vitro to study changes of a given GPCR during agonist induced receptor activation. As a result one can reason that stimulation of G protein-coupled receptors by an agonist leads to a conformational change and hence to a transition of the receptor into an active conformation, which can subsequently couple to a G-protein. Conformational changes have been well established to occur within the transmembrane domain (TM) III and VI and could have ramifications into the 3rd intracellular loop, which is one of the coupling regions for G-proteins. Evidence is accumulating that agonists of different efficacy could induced different changes in receptor conformations. Using a Fluorescence resonance energy transfer (FRET) based approach employing the FlAsH-tetracystein tag technology in combination with the cyan fluorescent protein (CFP), we were able to site-specifically label appropriately modified GPCR in living cells(1). Since the tag consists of minimally only six amino acids we were able to position the labeling sequence in different positions within the 3rd intracellular loop of the α2A- adrenergic receptor. Here we report, for the first time in living cells, differences in conformational changes in specific locations of the third intracellular loop when a receptor is stimulated with full or partial agonists. The positions were chosen to be underneath transmembrane domain (TM) VI, underneath TM V or in the middle of the 3rd intracellular loop, while CFP was fused to the C-terminus. All constructs were characterized with respect to ligand binding and no significant changes were observed when compared to the wild-type receptor. All receptors constructs were expressed at the cell surface and showed agonist induced changes in the FRET-ratio when stimulated with the full agonist norepinephrine. Significant differences were observed for a set of ligand with different efficacy for G-protein stimulation. The strong partial agonists, dopamin and clonidine, both induced changes in the FRET signal for all three receptor constructs tested, but with significantly smaller signal amplitude than the full agonist norepinephrine. The weak partial agonists, norphenylephrine and octopamine, only induced changes in the FRET-signal for the construct underneath TM VI. From these data it can be speculated, that strong partial agonists, can induce conformational changes within the entire 3rd intracellular loop, while weak partial agonists do only induce conformational changes close to TM VI.