Lasting increases in the population spike recorded in area CA1 of hippocampal slices may be evoked by the patterned presentation of as few as 5 stimulus pulses delivered to the commissural/associational afferents. The most effective pattern of stimulus presentation was a single priming pulse followed 170 ms later by 4 pulses at 100 Hz; control trains of 5 pulses at 100 Hz had no significant enduring effect. This pattern-dependent phenomenon, termed primed burst potentiation, is of lesser magnitude than is the long-term potentiation induced by 100 Hz/l s stimulation, but appears to show a similar time course and duration. In addition, the two phenomena are not additive, suggesting that they may share a common mechanism.