Abstract Adolescence is a vulnerable period of postnatal development for the onset of major psychiatric disorders where the prefrontal cortex (PFC) is involved. According to the NIMH Council's Workgroup Report, many psychiatric disorders can only be understood as an interaction between brain development and susceptibility to risk factors. Although many progresses have been made in the field during the past few years, a comprehensive understanding of the cellular and circuit level mechanisms regulating the developmental trajectory of neural processes involved in these disorders remains incomplete. Thus, our long-term goal is to identify sensitive developmental processes during adolescence that contribute to the onset of psychiatric disorders where the PFC is compromised. From studies performed during the preceding grant period, we have found that a hallmark of PFC maturation during adolescence is the functional re-calibration of an excitatory-inhibitory (E-I) balance state. In addition to the gain of GABA function, there is a facilitation of GluN2B and GluN2A NMDAR transmission in the PFC that is intimately linked to ventral hippocampal and basolateral amygdalar inputs. Yet, the extent to which coordinated activation of ventral hippocampal and basolateral amygdalar inputs during adolescence are required for the functional maturation of the PFC remains unclear. We will fill this gap in knowledge through the pursuit of 3 Specific Aims. We will use an input-specific chemogenetic strategy to transiently inhibit PFC afferent transmission at 3 non-overlapping adolescent periods to establish the exact window(s) of susceptibility for the gain/functional maturation of the GABA and NMDA synaptic components of the PFC E-I balance and their impact on PFC-dependent behaviors in adulthood. Together, the proposed aims are expected to uncover key neural circuit processes that contribute to the enhanced PFC vulnerability during adolescence to developmental insults. Such knowledge is expected to provide insights on the implementation of new therapeutic interventions to prevent/mitigate the incidence of cognitive and affective deficits often seen in mental illnesses that emerge during adolescence.