The early developmental period is characterized by a high degree of plasticity and, consequently, is very sensitive to environmental factors, such as early life stressors (ELS). Exposure to ELS is known to increase risk to psychopathologies such as depression and anxiety disorders later in life . At a cellular level, alterations in the migration and integration of GABAergic interneurons (INs) in cortical circuits have emerged as a key processes involved in the vulnerability to psychiatric disorders . In humans and rodents, ELS interacts with genes regulating the serotonin system to increase risk to stress-related disorders . In addition, ELS is associated to a variety of epigenetic methylation changes in blood DNA from patients displaying a high loading of ELS . Here, we aimed to investigate the role of the ionotropic serotonin 3A receptor (5-HT3AR) at a genetic and epigenetic level in rodent and human models of early-life stress. We will first present data indicating that the 5-HT3AR is specifically expressed in a subset of cortical INs derived from the caudal ganglionic eminence (CGE) and controls early steps of cortical circuit assembly . Interestingly, the migration, transcriptional programs and positioning of 5-HT3AR expressing interneuron subtypes were found to be dysregulated in pathological models of early-life serotonin dysregulation. At a behavioral level, we found that ELS interacts with the 5-HTR3A to modulate social behaviors. Finally, we will present human data indicating that childhood adversity significantly impacts the methylation status of the promoter region of the human 5-HT3AR in an allele-specific manner. Taken together, this presentation will highlight the importance of the serotonin system in early life development and psychopathology with a special focus on the role of the 5-HT3AR in cortical interneuron development.