A high content screening (HCS) assay for rapidly screening one or more compounds to determine functional response or pharmacological properties thereof, comprising: i) priming a cell or cell material with a sensor for a biological response; ii) contacting the compound(s) to be tested with the primed cell or cell material or contacting a cell or cell material which has been contacted with the compound(s) with the primed cell or cell material; iii) simultaneously or subsequently contacting with a fluorescent agonist or a fluorescent neutral antagonist wherein the binding of the fluorescent agonist or antagonist and its associated biological response are detected or monitored in the same cell and are distinct allowing separate readout.