Previously the involvement of platelet-activating factor (PAF) as a retrograde messenger in the induction of long-term potentiation (LTP) in CA1 region of rat hippocampus has been reported (Kato, K. et al. (1994) Nature 367, 175-179). In this report, the effect of PAF receptor antagonist on LTP in the dentate gyrus has been demonstrated and suggests the existence of common mechanisms of LTP in Ca1 and dentate gyrus. The site responsible for the expression of LTP induced by PAF has also been examined. The frequency of miniature EPSCs were enhanced by application of a PAF analog, but median amplitude were unaffected, suggesting that PAF preferentially affects presynaptic sites. Evoked synaptic responses were also monitored and the potentiation by the PAF analog was observed. Interestingly, there is a discrepancy between the results observed in the miniature and evoked potential studies in terms of dose effect of PAF. This discrepancy could have resulted from repeated synaptic stimulations required for studying evoked responses. The requirement for a monitoring stimulation in the evoked response measurement could activate additional biological process that enhance LTP expression.