G protein-coupled receptors (GPCRs) are heptahelical, transmembrane proteins that mediate a plethora of physiological functions by binding ligands and releasing G proteins that interact with downstream effectors. GPCRs signal as monomers, complexes of the same receptor subtype (homomers), or complexes of different receptor subtypes (heteromers). Recently, heteromeric GPCR complexes have become attractive targets for drug development since they exhibit distinct signaling and cell-specific localization from their homomeric counterparts. Yet, the effect of heteromerization on the pharmacology of many GPCR homomers remains unknown. Therefore, we have undertaken the task to examine the effect of heteromerization on Gs signaling through the adenosine 2A receptor (A2AR) and Gi signaling through the dopamine type 2 receptor (D2R) since the A2AR-D2R heteromer is an emerging therapeutic target for Parkinson’s disease (PD). We examined the effect of heteromerization on A2AR and D2R homomeric signaling using electrophysiology and the Xenopus laevis oocyte heterologous expression system. G protein-coupled inwardly rectifying potassium channels (GIRKs) were used as reporters for Gi signaling because activation leads to direct Gbeta-gamma (Gβγ)-mediated stimulation of the GIRK current. We also coupled GIRK channels to Gs signaling by overexpressing Gαs and signaling throughGαsβγ. Our electrophysiological assay is innovative because it allows us to optimize the conditions of heteromerization and directly observe GPCR signaling at the G protein level. Our data demonstrate that heteromer formation alone decreases dopamine-elicited Gi signaling through the D2R and CGS-21680-elicited Gs signaling through the A2AR. Furthermore, this reciprocal antagonism was predominately due to changes in efficacy versus potency. We also examined crosstalk observing that applying agonists or antagonists to the adjacent receptor further modulate this inhibition with the combination of agonists and antagonists relieving inhibition. Mutating the A2AR-D2R heteromer interface abrogated all of the aforementioned ligand-induced effects on G protein signaling through the A2AR-D2R heteromer. We are currently aiming to validate our results from the oocyte experiments with an in vivo model. Our data further elucidate the effect of various ligands on G protein signaling through the A2AR- D2R heteromer, which may facilitate future studies that examine A2AR-D2R heteromer signaling.