G-protein coupled receptors (GPCRs) implicated in disease are the predominant pharmaceutical targets. Growing evidence suggests that GPCRs form homo- and heteromeric complexes, resulting in allosteric functional changes. Ligands targeting one receptor can alter the function of the other receptor or receptors. Knowledge of these functional changes will provide unique opportunities to treat diseases. We examined two GPCR heteromers implicated in psychosis: mGlu2R-5HT2AR and D2R-5HT2AR. Using whole-cell patch clamp, we studied HEK-293 cells stably transfected with mGlu2R and 5HT2AR. Maximal heteromer formation allows inverse agonists to increase the G-protein activity of the opposite receptor, while sub-maximal heteromer formation does not. However, similar results are obtained in sub-maximal heteromer cells by applying a combination of a mGlu2R synthetic agonist with a 5HT2AR anti-psychotic drug. These results confirm our oocyte results, now in a mammalian cell line. Using two-electrode voltage clamp, we also investigated the allosteric changes upon heteromerization of D2R-5HT2AR in oocytes injected with appropriate cRNAs. Heteromer formation in the presence of dopamine or serotonin results in an increase in G-protein activity of each receptor while the simultaneous presence of both neurotransmitters further increases the G-protein activity. The addition of synthetic agonists or anti-psychotics decreases the G-protein activity of the opposite receptor while agonizing or antagonizing its target receptor, respectively. Maximal allosteric effects upon D2R-5HT2AR formation only occur at a specific cRNA injection ratio, but partial effects exist at other ratios. Our data suggest that allosteric functional changes upon heteromerization are physiologically relevant and are mostly different when comparing mGlu2R-5HT2AR to D2R-5HT2AR.