Background: There is cross-talk between serotonin and melatonin hormones. Results: There is evidence for unidirectional transactivation and a heteromer-specific signaling profile for formation of functional melatonin MT 2 and serotonin 5-HT 2C receptor heteromers. Conclusion: A new potential target of the antidepressant agomelatine is identified. Significance: The importance of binding of multitarget drugs to GPCR heteromers in psychiatric disorders is demonstrated.