The effects of median raphe nucleus (MRN) stimulation on the induction of long-term potentiation (LTP) was investigated in the dentate gyrus (DG) of urethane-anaesthetized rats. LTP of both DG population spike amplitude and population EPSP slope was induced when tetanic electrical stimulation (100 Hz, 1 s) of the perforant path (PP), at an intensity which did not produce significant LTP alone, was presented concurrently with tetanic stimulation of the MRN. Tetanic stimulation of either the PP alone or of the PP and MRN together did not affect the short-term enhancement of population spike amplitude produced by single stimuli to the MRN. These findings suggest that serotonergic afferents from brainstem raphe nuclei may modulate the induction of LTP in the dentate gyrus.