Classically, G protein-coupled receptor activation by a ligand has been viewed as producing a defined response such as activation of a G protein, activation or inhibition of adenylyl cyclase, or stimulation of phospholipase C and/or alteration in calcium flux. Newer concepts of ligand-directed signaling recognize that different ligands, ostensibly acting at the same receptors, may induce different downstream effects, complicating the selection of a screening assay. Dynamic mass redistribution (DMR), a label-free technology that uses light to measure ligand-induced changes in the mass of cells proximate to the biosensor, provides an integrated cellular response comprising multiple pathways and cellular events. Using DMR, signals induced by opioid or cannabinoid agonists in cells transfected with these receptors were blocked by pharmacologically appropriate receptor antagonists as well as by pertussis toxin. Differences among compounds in relative potencies at DMR versus ligand-stimulated GTPγS or receptor binding endpoints, suggesting functional selectivity, were observed. Preliminary evidence indicates that inhibitors of intermediate steps in the cell signaling cascade, such as receptor recycling inhibitors, mitogen-activated protein kinase kinase/p38 mitogen-activated protein kinase inhibitors, or cytoskeletal disruptors, altered or attenuated the cannabinoid-induced response. Notable is the finding that mitogen-activated protein kinase kinase 1/2 inhibitors attenuated signaling induced by the cannabinoid type 2 receptor inverse agonist AM630 but not that stimulated by the agonist CP 55,940. Thus, DMR has the potential to not only identify ligands that activate a given G protein-coupled receptor, but also ascertain the signaling pathways engaged by a specific ligand, making DMR a useful tool in the identification of biased ligands, which may ultimately exhibit improved therapeutic profiles.