The hippocampus is rich in both glucocorticoid receptor (GR) and neuronal nitric oxide synthase (nNOS). But the relationship between the two molecules under physiological states remains unrevealed. Here, we report that nNOS knockout mice display increased GR expression in the hippocampus. Both systemic administration of 7-Nitroindazole (7-NI), a selective nNOS activity inhibitor, and selective infusion of 7-NI into the hippocampus resulted in an increase in GR expression in the hippocampus. Moreover, KCl exposure, which can induce overexpression of nNOS, resulted in a decrease in GR protein level in cultured hippocampal neurons. Moreover, blockade of nNOS activity in the hippocampus leads to decreased corticosterone (CORT, glucocorticoids in rodents) concentration in the plasma and reduced corticotrophin-releasing factor expression in the hypothalamus. The results indicate that nNOS is an endogenous inhibitor of GR in the hippocampus and that nNOS in the hippocampus may participate in the modulation of Hypothalamic-Pituitary-Adrenal axis activity via GR.