The invention is directed to a method for identifying substances acting as ligands for transfected receptors by using transfected markers to measure receptor/ligand interactions. The present invention also relates to a method of identifying compounds which act as inverse agonists of the 5-HT2A receptor, the method comprising contacting a constitutively active 5-HT2A receptor with at least one test compound and determining any decrease in the amount of basal activity of the receptor so as to identify a test compound which is an inverse agonist of the 5-HT2A receptor. Such inverse agonists may be used in the treatment of schizophrenia and related psychoses.