The molecular basis of vulnerability to stress during the adolescent period is largely unknown. To identify potential molecular mediators that may play a role in stress-induced behavioral deficits, we imposed social isolation on a genetically vulnerable mouse model.We report that 3-week (5-8weeks of age) adolescent stress in combination with disrupted-in-schizophrenia 1 (Disc1) genetic risk elicits alterations in DNA methylation of a specific set of genes, tyrosine hydroxylase, brain-derived neurotrophic factor and FK506 binding protein 5. The epigenetic changes in the mesocortical dopaminergic neurons were prevented when animals were treated with a glucocorticoid receptor (GR) antagonist RU486 during social isolation, which implicates the role for glucocorticoid signaling in this pathological event. We define the critical period of GR intervention as the first 1-week period during the stress regimen, suggesting that this particular week in adolescence may be a specific period of maturation and function of mesocortical dopaminergic neurons and their sensitivity to glucocorticoids. Our study may also imply the clinical significance of early detection and prophylactic intervention against conditions associated with adolescent social stress in individuals with genetic risk.