The present invention relates to an in vitro method for detecting cAMP or cGMP comprising a) contacting a mixture with a complex of a tracer and a dequencher, wherein the tracer is a fluorophore covalently linked to a cAMP quencher, and b) measuring the change in fluorescence. Furthermore, the present invention relates to the use of said method for determining the cAMP or cGMP concentration in the mixture, for determining the activity of a receptor wherein the signal transduction of the receptor comprises cAMP or cGMP and for screening of a ligand for such a receptor.