The phasic effects of stress-induced elevations of corticosterone on long-term potentiation (LTP) were investigated in the hippocampus of awake, freely behaving rats. Field potential recordings were performed in the dentate gyrus with stimulation of the medial perforant pathway or the CA1 with stimulation of the commissural/associational pathway, on the contralateral hemisphere. LTP was induced either shortly (1 h) after acute stress or 4 h later. Animals were either adrenalectomized or adrenally intact. A subgroup of animals were injected with a low dose of dexamethasone 4 h prior to the stressor, in order to suppress the corticosterone response to restraint stress, and they were tested for LTP in the dentate gyrus 4 h after the stressor. In the dentate gyrus, stress had no effect on LTP induction at 1 h post-stress; however, it produced a significant suppression at the 4 h interval. As expected, adrenalectomized rats did not show stress-suppression of LTP, but showed a lower level of LTP with or without stress. Supporting a role of stress-induced glucocorticoid secretion in LTP suppression, dexamethasone treatment of adrenally intact animals blocked the acute stress suppression of LTP in the dentate gyrus. In the CA1 field, restraint stress did not significantly affect LTP induction at either the 1- or 4-h post-stress intervals. Similarly, stress by itself, did not significantly affect neuronal excitability in either the dentate gyrus or CA1 hippocampal field at either the 1- or 4-h post-stress interval. The present results suggest that stress affects synaptic plasticity differently at the two hippocampal subfields and that the effects are time-dependent and involve the stress-induced surge of glucocorticoids.