The effect of acute corticosterone (CORT) treatment on elaboration of long-term potentiation (LTP) and short-term potentiation (STP) in intact and adrenalectomized rats was investigated. Both forms of synaptic plasticity were evaluated as fractional increases in population spike amplitude in dentate gyrus granule cells following brief, high-frequency stimulation of the medial perforant pathway in vivo. LTP was distinguished from STP by the magnitude and time course of population spike facilitation. Adrenalectomy resulted in profound reduction of LTP extent, without any significant effect upon STP. Replacement of glucocorticoid by a single injection of CORT to previously adrenalectomized rats restored LTP to control levels. When CORT was applied to intact subjects, it slightly, but significantly, impaired LTP development. No effect of CORT on STP induction and time course was found. Taken together, these results show that the modulation of synaptic efficiency in hippocampal dentate gyrus by circulating glucocorticoid depends on the form of plasticity examined. LTP was found to be more susceptible to the impairment induced by both high and low levels of glucocorticoid than STP.