Norepinephrine in the hippocampus of urethane anesthetized rats was monitored by microdialysis prior to, and following, an intracerebroventricular injection of 5 micrograms [-]NE in 2 microliters of ACSF. The perforant path evoked potential in dentate gyrus was concurrently monitored by a microelectrode adjacent to the dialysis probe. NE levels 30 x basal values or greater at the recording site were related to long-lasting potentiation of the perforant path-evoked population spike. Increases of 3 x to 6 x basal NE values were not accompanied by any potentiation. NE in the midline ventricles alone did not produce potentiation. These results suggest effective NE concentrations for beta-receptor activation of long-term potentiation would only occur near the NE release site.