Research in the past decade has shown that G-protein-coupled receptors (GPCRs) can form oligomeric structures, and that oligomerisation of different GPCRs can alter their properties and the signals they induce. On page 2486, Evgeni Ponimaskin and co-workers now demonstrate that the serotonin receptors 5-HT1A and 5-HT7 are able to homo- and hetero-dimerise, and they analyse the functional consequences of heterodimerisation. By employing a quantitative lux-Förster resonance energy transfer (FRET) technique and mathematical modelling, they find that the relative amount of the heterodimer that forms in the plasma membrane depends on the expression ratio of the 5-HT1A and 5-HT7 receptors. The formation of heterodimers increases the internalisation rate of 5-HT1A and alters the signals downstream of this receptor: heterodimerisation results in a weaker activation of the inhibitory Gi protein and a stronger activation of extracellular signal regulated kinase (ERK1/2). Furthermore, it reduces the ability of the 5-HT1A receptors to activate K+ channels in neurons. In addition, the authors show that the expression level of 5-HT7 receptors in the hippocampus decreases throughout development, which indicates that heterodimerisation of these two neurotransmitter receptors probably has important physiological functions during brain development.