Increased level of glucocorticoid may be related to the pathophysiology of depressive disorder. The involvement of BDNF (brain-derived neurotrophic factor) in the antidepressive effect has also been suggested, however, the possible influence of glucocorticoid on the action of BDNF in the developing central nervous system (CNS) has not been elucidated. In this study, we investigated the effect of glucocorticoid (Dexamethasone, DEX) on synaptic maturation and function enhanced by BDNF in early developing hippocampal neurons. In the immature stage, BDNF increased the outgrowth of dendrites and the expression of synaptic proteins including glutamate receptors and presynaptic proteins. Pretreatment with DEX significantly inhibited the BDNF-dependent up-regulation of both dendritic outgrowth and synaptic proteins. In the more mature stage, the BDNF-reinforced postsynaptic Ca²⁺ influx was decreased by DEX. BDNF-enhanced presynaptic glutamate release was also suppressed. RU486, glucocorticoid receptor (GR) antagonist, canceled the DEX-dependent blocking effect on the action of BDNF. After down-regulation of GR by siRNA application, no inhibitory effect of DEX on the BDNF-increased synaptic proteins was observed. Interestingly, the BDNF-activated MAPK/ERK (mitogen-activated protein kinase/extracellular signal-regulated protein kinase) pathway, which is an essential intracellular signaling for the BDNF-increased synaptic proteins, was reduced by DEX. These results suggest that BDNF-mediated synaptic maturation is disturbed after neurons are exposed to high-level glucocorticoid in their development stage.