ABSTRACT Serotonin regulates multiple physiological and pathological processes in the brain, including mood and cognition. The serotonin receptors 5-HT1AR (also known as HTR1A) and 5-HT7R (also known as HTR7) have emerged as key players in stress-related disorders, particularly depression. These receptors can form heterodimers, which influence their functions. Here, we explored the developmental dynamics of 5-HT1AR and 5-HT7R expression and validated heterodimerization levels in the brain of control and stressed mice. In control animals, we found that there was an increase in 5-HT1AR expression over 5-HT7R in the prefrontal cortex (PFC) and hippocampus during development. Using a chronic unpredictable stress as a depression model, we found an increase in 5-HT7R expression exclusively in the PFC of resilient animals, whereas no changes in 5-HT1AR expression between control and anhedonic mice were obtained. Quantitative in situ analysis of heterodimerization revealed the PFC as the region exhibiting the highest abundance of 5-HT1AR–5-HT7R heterodimers. More importantly, upon chronic stress, the amount of heterodimers was significantly reduced only in PFC of anhedonic mice, whereas it was not affected in resilient animals. These results suggest an important role of brain-region-specific 5-HT1AR–5-HT7R heterodimerization for establishing depressive-like behaviour and for development of resiliency.