Fluorescence spectroscopy is the most common non-radioactive technique used to study GPCR interactions with their ligands. Raster image correlation spectroscopy (RICS) exploits spatio-temporal correlation functions rather than the simple temporal correlations of conventional fluorescence correlation spectroscopy. In this paper we describe the use of RICS and the number and brightness method to determine the diffusion of a construct of endothelin ETA receptor with EGFP and the aggregation state in the cytoplasm. Our construct seems to locate mainly in the cytoplasm where it undergoes diffusion and it appears to be monomeric. Although our construct could not fully represent the native protein, we believe that the methodology we describe in this paper could be used by anyone in this field.