1. We recorded the extracellular excitatory postsynaptic potentials (EPSPs) in CA1 in the freely behaving rats after stimulation of the apical dendritic and basal dendritic afferents. The apical dendritic population EPSP was negative at the apical dendritic layers and positive at stratum oriens and alveus. The basal dendritic population EPSP was negative at basal dendritic layer (stratum oriens) and positive at the cell body and apical dendritic layers. 2. We delivered various tetanic stimulus patterns in an attempt to elicit long-term potentiation (LTP) at the apical dendritic synapse of CA1 pyramidal cells. A 200-Hz high-frequency train (HF) 1 s in duration was more successful than theta-frequency primed bursts (PBs; 1 or 4 priming pulses followed by 10 or 4 pulses at 100 Hz) in eliciting LTP of the apical dendritic population EPSP, recorded either ipsilaterally or contralaterally. 3. Tetanus applied to the apical dendritic afferents occasionally elicited long-term depression (LTD) at an intensity lower than the LTP threshold. LTD persisted to > or = 1 day after a tetanus. 4. High-intensity tetanic stimulations often elicited hippocampal afterdischarges (ADs). If the AD duration was brief (< 15 s), apical dendritic LTP could follow the AD. However, if the AD duration was long, no apical dendritic LTP was observed. Tetanus that evoked an apical dendritic LTP typically evoked a short hippocampal AD < 15 s in duration. 5. LTP of the basal dendritic population EPSP was readily elicited either after an HF or a PB tetanus of low (1.5-2 times threshold) stimulus intensity.(ABSTRACT TRUNCATED AT 250 WORDS)