Hippocampal slices from norepinephrine (NE)-depleted rats exhibited marked reductions in long-term potentiation (LTP) of both the population spike and dendritic EPSP in the dentate gyrus. In contrast, depletion of serotonin (5-hydroxytryptamine, 5-HT) had no effect on either population spike or EPSP-LTP. In addition, superfusion of slices with NE produced potentiation of both the granule cell population spike and dendritic EPSP which persisted long after NE washout. These data support a role for NE in regulating long-term plasticity of both granule cell action potential firing and dendritic EPSPs.