Use of cell-substrate impedance based methods for screening for agonists of G-Protein Coupled Receptors (GPCRs) or inhibitors of a Receptor Tyrosine Kinases (RTKs), identifying compounds that affect GPCR or RTK pathways, validating molecular targets involved in a GPCR or RTK signaling pathways, monitoring dose-dependent functional activation of GPCR or RTK; determining desensitization of a GPCR and identifying a compound capable of affecting RTK activity in cancer cell proliferation.