An increasing number of G-protein-coupled heptaspanning-membrane receptors (GPCRs) are reported to be expressed on the plasma membrane as dimers. Quite often dimers are constitutive in a given cell and a given cell can express both homodimers and a variety of heterodimers. From our work on homo and heteromerization of receptors present in striatal neurons it has been feasible to demonstrate some consequences of homo and heteromerization in terms of both, ligand pharmacology and receptor function. There are several examples showing that cells sense neurotransmitters (or hormones) in a different way when they express one or another set of receptors that ensemble into heterodimers (see for instance Ginés et al, 2000; Canals et al., 2003). One of such examples is given by the lack of desensitization of D1 receptors (D1R) interacting with adenosine A1 receptors (A1R) in striatal GABAergic neurons unless both receptors in the D1R/A1R heteromer are activated simultaneously. In fact signalling and/or desensitization can vary depending on whether one or the two receptors of the heteromer are activated and the resulting signal is quite often is not simply the addition of the signals given by individual activation of the receptors; even there are instances in which heteromer-mediated signalling becomes qualitatively different. On the other hand homo or heteromerization leads to the so-called intramembrane (or horizontal) interactions, which means that the pharmacology of a given receptor usually changes (for agonists and/or antagonists): i) when it forms heteromers with another receptor and/or ii) when the partner receptor in the heteromer is activated. This is due to conformational changes in the receptors transmitted within the receptor-receptor interface at the plane of the membrane bilayer. In this regard new models considering receptor dimers have been recently developed (Albizu et al., 2006, Franco et al., 2005, 2006). Until now the approaches for fitting ligand binding data have been based on the existence of receptor monomers. From a recently devised model for receptor dimers, there exists a new approach for fitting data that gives more accurate and physiological relevant parameters (Casadó et al., 2007, data in preparation). Fitting data using the new procedure gives not only the equilibrium dissociation constants for high and low affinity binding to receptor dimers but a new parameter reflecting the molecular communication within the dimer. A comprehensive way to fit binding data from saturation isotherms and from competition assays to a dimer receptor model is now possible as it is also possible to give actual values for the concentration giving 50% reduction in radioligand binding when performing competition experiments. These values are much more reliable to establish potency orders in Pharmacology than the IC50 values reported from competition experiments using monomer-based approaches. In summary the occurrence of receptor dimers opens for GPCRs new perspectives from both the functional and the pharmacological point of view.